Soichi Isurugi

Soichi Isurugi (石動 惣一) is the father of Misora Isurugi, the owner of the café nascita and one of the main villains in Kamen Rider Build. Upon finding the amnesiac Sento Kiryu, he gave him a place to stay in the basement of nascita. He is also the one responsible for creating the Sky Wall Disaster and is the true identity of Blood Stalk (ブラッドスターク), a general and later the leader of Faust who “killed” Takumi Katsuragi (later revealed to be actually Taro Satou and has his corpse disguised as Takumi) and framed Ryuga Banjou for the murder.

Early Life
Ten years prior to the events of Build, Soichi was an astronaut working with the Kiwami Project, discovering the Pandora Box on Mars. However finding the box would change his mentality, which he jokingly admitted that it "did a number on his mind". At the ceremony back on Earth, he ran toward the Pandora Box and triggered it despite a few guards attempting to restrain him, causing the Sky Wall Disaster. After a series of later psychological attempts, Soichi was arrested and placed in a mental hospital, but he escaped about a month later. Around the same time he was admitted into the hospital, Misora Isurugi was also admitted as well due to the exposure of the box's affects and gained the ability to purify Fullbottles. At some point he opened up a café called nascita and took residence in Touto. In the following seven years, he was appointed as a special consultant for the Pandora Box, and was asked by Namba Heavy Industries Ltd. to help oversee Faust's activities, all while receiving a Transteam Gun and becoming Blood Stalk. Around this time, Misora joined Faust to purify the Fullbottles for them, not knowing it was for evil purposes.

After Misora eventually found out about Faust's evil intentions, she lost her will to do so, so Soichi "saved" her and escaped with her back to the cafe, where Soichi convinced her to purify the bottles for good. He stole two of the Pandora Box panels from the Touto Institute of Advanced Matter Physics to ensure their confidence. From Faust, he took along a Build Driver and an unknown number of Fullbottles; upon his return, he plastered the panel into one of nascita's basement walls.

In two years, after learning of Katsuragi's, one of the creators of Faust, desire to quit, Isurugi confronted him in his apartment as Blood Stalk. However, he was prepared for this scenario as Taro Satou, a punk rockstar, arrived at Katsuragi's apartment complex for a job interview. When Katsuragi remained adamant on his decision to leave Faust, Stalk knocked him out before Taro arrived in the room, completely horrified when he saw Stalk and an unconscious Katsuragi on the floor. As Taro scrambled frantically to the door, Stalk tossed a knife in his back, killing him instantly. Standing over the body, Stalk displayed an ability he obtained during his mission to Mars: the ability to alter a person's face. He turned Taro's face into Katsuragi's, and Katsuragi's to Taro's before switching their clothes and reposition Taro's body before dragging the unconscious Katsuragi outside and into a car. After cancelling his transformation, Isurugi moved to the next phase of his plan as Ryuga Banjo, an ex-fighter excommunicated for cheating, entered the apartment complex, once again for seeking employment. Before Ryuga arrived, Isurugi made an anonymous call to the police, stating his neighbor, Katsuragi was stabbed and killed by Ryuga, and then waited for the cops to arrive an apprehend Ryuga. Isurugi then wiped Katsuragi's memory, another ability obtained on Mars, and took the amnesiac Katsuragi to Faust's lair.

As both Stalk and his companion Night Rogue watched over Katsuragi's enfusion of the Nebula gas, with the latter completely unaware of Katsuragi's true identity, both Stalk and Rogue were surprised that Katsuragi didn't turn into a Smash after being infused with the element, only losing his memory of the lair's location. They both decided to see what would happen to a person containing the Smash element without turning into a Smash. On September 5th, he brought the unconscious Katsuragi to a small alleyway on a rainy day. While waiting for Katsuragi to wake up, he cancelled his transformation and went to buy an umbrella before coming back to a now awake Katsuragi. Nurturing him until he was better, Isurugi presented the Build Driver to the amnesiac Katsuragi, telling him he can defeat the Smash creatures with it. Katsuragi was still unclear on what a Smash was, but Isurugi urged him to help them.

Returning from his first encounter with a Smash, Katsuragi was greeted by Isurugi and Katsuragi accepted it. From then on, Isurugi renamed him Sento Kiryu and nurtured him into becoming Kamen Rider Build to help him recover his memories and uncover the bat-like man, but in reality all to ensure that as Build continues fighting more monsters, and so Misora can purify the bottles for justice.

The RabbitTank and Dragon's Intertwined Fate
The morning following Kamen Rider Build, whom has been active for a year, defeated the Needle Smash and saved a freelance journalist, Isurugi was sweeping the front entrance of nascita and proceeded inside when finished until he heard Sento emerging from the secret lab entrance. He hid until Sento fully emerged and greeted him with surprise, and examined the newly created Harinezumi Fullbottle, congratulating Misora for her part in the bottle's purification. Part of his ruse, Isurugi asked Sento if any of his memory came back, to which Sento stated the same few fragments involving a scientist with a gas mask, human testing and a bat-like man. He then amusingly gave Sento his rent, much to the amnesiac's surprise as he had no knowledge of Isurugi charging him to stay at nascita, then surmised it was the reason Isurugi made Sento take the job position at Touto's Institute of Advanced Matter Physics. Isurgi then reminded Sento of this day being his first at the job, realizing he's late. Before he left, Sento unveiled his new invention: the Build Phone and its ability to turn into the Machine Build motorcycle after combining the phone with the Lion Fullbottle. Amazed by his own invention, Sento prepared to drive to work until Isurugi reminded him he's still inside the cafe.

Later on, he learned that one of Faust's test subjects, Ryuga, whom also didn't turn into a Smash after being administered with the Nebula Gas, escaped and informed Sento. Inside the hidden lab, Isurugi gave all the information of Ryuga's background to Sento over the Build Phone. He also stated the scanners for Smash had picked him out and hacked the motorcycle Ryuga was using to gain his coordinates, relaying the location to Sento, knowing full well Gentoku had learned of Ryuga's escape and would send a Smash to intercept and retrieve Rygua.

After learning of making himself an enemy of Touto and bringing Ryuga to their secret lab at night, Isurugi became upset and scolded Sento for this current dilemma he's put himself in. Sento then explained to Isurugi of Ryuga's experimentation by Faust appears similar to Sento's memory and how Sento himself could've been a test subject. To keep up his con, Isurugi then stated if they were both experimented on then they would've both turned into Smash, and how Ryuga didn't lose his memory, leading to the possibility of them finding Faust's lair. However, Sento stated Ryuga can't remembered where the base was, but came up the theory of finding the base by clearing Ryuga's name and finding the true murderer. Isurugi and Sento approached Ryuga, asking him to tell them everything, until Sento and Ryuga argued as Ryuga began his tale from birth while Sento meant the details around Ryuga's incarceration, annoying Isurugi as he walked to his rocking chair by the stairs. As Ryuga told them the cops arrived as he entered the scientist's apartment for a job interview, Isurugi objected to Ryuga's claim, claiming it to be convenient for the cops to arrive at the same time Ryuga found the body. Ryuga claimed it to be the truth until the silver booth went off, unveiling the purified Gorilla Fullbottle. Isurugi rejoined the group as Misora met Ryuga and Sento proclaimed Ryuga' arrest and jailbreak may be connected until they spotted Sawa Takigawa entering the room, completley unaware that she's one of Juzaburo Namba's children and a spy for Faust. Misora punched Isurugi in the shoulder for leaving the door open, but Isurugi stated he closed the door but realized he left it open, another ruse to kep up his con. He pretended to be even more shocked while Sento was surprised that Sawa learned he was Kamen Rider Build, presenting the nascita match box he dropped the previous night, unaware she took it from the bowl on the counter before entering the secret door. She then claimed she was there to thank Sento, and to want to do a cover story on the Kamen Rider, but Sento declined until Isurugi agreed, much to the annoyance of Sento and Misora.

In the morning, Sento, as Build, responded to a Smash alert, unaware it was a trap to lure him out by Faust. At the location, Build phoned Isurugi to let him know there wasn't a Smash in sight, but Isurugi told him the reports picked up a Smash, until Build realized he walked into a trap by the Touto government sent to arrest him. After Sento and Ryuga's encounter with Night Rogue and Ryuga's fiance, Isurugi sat outside of Nascita reading the Touto Info. on his tablet. Isurugi deemed Sento's fugitive status won't interfere in his plan to purify all of Touto's Fullbottles. He then developed an itch in his back and transmutated his tablet into a glass back scratcher and scratched his back with it, relieving his itch.

Return of the Cobra
Overhearing Sento's explanation of how Masahiro Nabeshima, the man whom gave the job interview with Katsuragi to Kasumi to give to Ryuga, and the same man whom knocked out Ryuga with a sedative as he was selected by Faust as a lab rat, Isurugi entered the lab with flare as Ryuga questioned Misora's capability of finding information on Nabeshima. He told Ryuga not to underestimate Misora and proceeded to help her change into her online idol alter-ego, Miitan. After turning on the camera, Isurugi explained to Ryuga that Miitan is a famous idol over the internet, with hundreds to thousands of fans submitting whatever information she ask. In an instant, they've received information, but of a Smash sighting. Isurugi remained in the lab to monitor any information on Nabeshima while he watched Ryuga argue with Sento as he prepared to head toward the Smash.

After Sento returned with the Flying Smash's essence and have Misora purify it into the Taka Fullbottle, Isurugi told Ryuga what Sento was doing as he placed the Taka and Soujiki bottles in a panel placed in a wall: he was looking for a Best Match. Isurugi said it twice jokingly, and told Ryuga that a pair of Fullbottles have their own affinity as he demonstrated with the Rabbit and Tank bottles, placing them in the panel to show their affinity. He also pointed out each Best Match has its own power, but finding two bottles to form a Best Match is difficult as he placed the bottles back on the table. Sento then joined the explanation, stating his Build Driver was originally an analyzer before it gained the transformation function. Ryuga decided to see if he can find a Best Match, which intrigued Isurugi but Sento doubted it until Ryuga placed the Taka and Gatling bottles in the driver, forming a Best Match. Sento was disbelieved while Isurugi congratulated Ryuga for his "sixth sense", and asked him how he did it, to which Ryuga pointed out that the Best Match can be found with an organic and inorganic bottles. As Sento argued with Ryuga on how to porperly find a Best Match, they accidently woke up Misora, whom threw a knife at the booth and twisted her sleeping mask tightly in her hands, whispering she'll cut them. Horrified, all three men apologized for waking her before Sento headed back to the touto Institute to find any information on Nabeshima while Ryuga was chained to one of the posts once more.

Near the night, Isurugi learned about Faust's trap to bring Ryuga to Seito by using Nabeshima's family as bait and decided to return as Blood Stalk. He made his way to the Touto Harbor Natsu Wharf, where Sento just defeated the Mirage Smash, which turned out to be Nabeshima. To keep him from waking Nabeshima, Stalk punctured Sento's back with one of his venomous tentacle, leaving him stunned under the poison's effect. Stalk then created a giant energy projection of a cobra and retrieved the incapacitated Nabeshima, leaving Sento on pass out on the ground after seeing Stalk near the smoke. After delivering Nabeshima back to Faust's lair, Stalk returned to the warehouse and brought the unconscious Sento back to nascita, waiting for the poison's effects to wear off before waking Sento by pinching both his cheeks at once before telling Sento he found him passed out from being poisoned and brought him back to nascita to recover. Sento told him it was a man dressed like Night Rogue, but with crimson and a cobra symbol. Before Sento could go any further, he received a call from Ryuga, learning that he and Sawa have made it to Seito. Isurugi heard Sento warning Ryuga that Nabeshima's family being captured by Faust could be a trap for Ryuga after telling him the Mirage Smash was Nabeshima himself. Isurugi asked Sento what he planned to do next after hanging up the phone, but instead Sento told him he gained a new memory fragment where he spotted a green panel with the same markings as the Pandora Box near Night Rogue as Sento was being prepped for the Nebula gas experiment. Sento then theorized that with the box's energy, Faust learned they can create superhuman beings, which Isurugi surmised, to keep up his con, that by solving the mystery surrounding the box would also expose Faust as well. They were interrupted by the silver booth going off, intriguing both Isurugi and Sento enough to see the new purified bottle, the Ninjya bottle. As Misora walked out of the booth and proceeded to bed, Sento asked her to gather information on the Skywall tragedy as Miitan, and after a few exchanges of "pleases" and "don't want to"s, Isurugi changed her clothes to Miitan's, forcing Misora to gather the information until she went to sleep.

After Sento received the information on the Skywall tragedy and returning from the Touto Institute earler than expected, Isurugi prepared to head to his part-time job until Sento brought up how Isurugi wanted him to become Kamen Rider Build, and over the past year Sento hasn't learned anything about Isurugi. Isurugi told Sento he doesn't eat pasta and has a fear of bidets before telling Sento to focus more on the present than the past and left, unaware that Sento learned that he was the astronaut whom brought the Pandora Box to Earth and caused the Skywall tragedy. As part of his plan, Isurugi sent a Smash report as Blood Stalk, stating specifically the subject is a second-dosed Smash, meaning if the element isn't extracted soon the person will die.

Watching Sento, as Build GorillaMond, fight against a Square Smash, Stalk fired a Devil Steam from his Steam Rifle, increasing the Smash's size to a giant's. As Build, in his RabbitGatling Trial Form, struggled against the giant Smash, Stalk informed him from afar that he's injected a new type of gas into the Square Smash, and even if Build saves the person inside, there will be lingering effects. As he watched Ryuga throw a Drill Crusher into the Smash's back, allowing Build to turn into his HawkGatling Best Match and perform a 100 Full Bullet finisher to defeat the Smash, Stalk takes his leave as Sento extracted the Smash essence from Nabeshima, whom was turned into a Smash once more by Rogue to see what would happened to a human being administered the Nebula gas a second time. Returning to the lair, Stalk was asked by Rogue, whom placed the Rocket bottle in their green Pandora Box panel, why he didn't kill Build as he fought the Square Smash. Stalk assured him as the game maker, he derived that not finishing Build will contribute to their overall scheme as time progresses. At night, Isurugi returned to hear a noise at the secret lab, and he rushed to see Sento smashing the wall with the bottle slot with a hammer. Isurugi attempted to stop Sento, but it was too late; the wall was damaged enough for Sento to remove the full slot, revealing it to be one of the green panels from the Pandora Box. Confused and angry, Sento asked Isurugi why he had it and if he's a member of Faust.

Ghosts of the Past
"You monster! Who's the one who injected me with the gas?! Who took away my memory?! Was it you?! Or was it Rogue?! Answer me! *Blood Stalk laughing* What's so funny?! Give me back my body! Give me back my memory!" "Your weakness is that you loss all judgement when it comes to your memory."

- Build going berserk while assaulting Stalk as he learned he was injected with the Smash element. Bringing Sento, Ryuga and Misora to nascita's main interior for a cup of coffee, Isurugi pretended to play dumb as he pour his newly made brew, irritating Sento to make him ask Isurugi again why he had one of the missing panels and if he's a member of Faust. Isurugi joked that he's a member of Faust, irritating Sento even more until Misora told Sento and Ryuga that she was captured by Faust because of her ability to purify the Fullbottles. Isurugi then added he stole the panel and a few bottles as he rescued Misora, all a lie to conceal his identity as Blood Stalk. Sento then unveiled of Isurugi as an astronaut from a decade ago, surprising Isurugi that Sento learned of his past as the astronaut whom retrieved the Pandora Box and opened it, causing the Skywall tragedy. Isurugi admitted the box messed with his mind, forcing him to open the box, still wondering why it made him do that. Now playing foosball with Sento, Isurugi admitted he joined Faust, another lie, to gain information about the organization and save Misora, then added that Faust could claw their way into the Touto Institute if Isurugi returned the panel to them. Sento asked why Isurugi didn't tell him and Isurugi stated he wanted Sento to become Kamen Rider Build to defeat Faust. Isurugi stated he wanted to make Faust answer to their crimes, but he and Misora alone couldn't make that happen. He then gave the reason why he didn't tell Sento everything relating to the panel: that Sento would not trust Isurugi, leaving Isurugi with the choice to let Sento discover the panel on his own. Buying his lies, Sento decided to forgive Isurugi, while telling him his coffee tasted horrible. Ryuga insulted Sento for being soft-hearted, but Sento explained that Isurugi woudln't put much effort in making horrible coffee if he wasn't telling the truth, leading to Isurugi, Ryuga and Misora to try Isurugi's brew, leaving the final verdict that it's horrible. As Isurugi told them they have no define palate, Sawa entered the room with evidence regarding Sento's past, which Isurugi decided it's time to put the next phase into motion.

After learning his true name to be Taro Satou from Tatsuya Kishida, a band member who was Taro's friend, Isurugi transformed into Blood Stalk and sent three Faust Guardians to attack a few contruction workers near the ally he placed the amnesiac Sento a year ago. As three Touto Guardians arrived and destroyed the Faust Guardians, Stalk took control of the Touto machines and had them attack the workers until Sento arrived as Build HawkGatling, destroying the corrupted robots. As he watched the workers run away, Stalk introduced himself propery to Build, Sawa and Tatsuya, saying his name. After Build realized he's the one turn Nabeshima into a Smash once more, Stalk praised him and gave a reward: a fight. Stalk and Build trade shots with their Hawk Gatlinger and Steam Rifle until Build managed to force Stalk off of a platform and onto the ground, continuing the fight as Build RabbitTank. Stalk blocked Build's Drill Crusher with his rifle, gaining the upper hand by placing his rifle at an diagonal angle to let the Drill Crusher's blade to roll don with it, allowing Stalk to kick Build. As Build executed a Vortex Finish Rider kick, Stalk cut his foot in midair, shocking Build as he was tossed aside. Stalk praised Build or his kick, but stated he has room for improvement before leaving.

Waiting in the dark at night, Stalk followed Tatsuya underneath a train bridge, listening how Tatsuya blaming himself for Sento's amnesia. With proficient speed, Stalk seized Tatsuya by the throat, squeezing hard until he passed out, bringing him to Faust's lair to be their next test subject. Laughing as he gaze at Tatsuya being prepped for the procedure, he took a spot to sit near Night Rogue, whom asked him why he chose Tatsuya. Stalk's response was to see if Tatsuya would be a worthy opponent for Build as Tatsuya is given the Nebula gas. After Tatsuya turned into the Press Smash, Stalk and Rogue let him loose, leading it to the Touto Institute, while Rogue headed there early as Gentoku to keep to his role in their plan. As Sento stood in a tunnel entrance to the parking lot, watched a squad of Touto Guardians firing at the Smash, Stalk appeared from behind, unveiling the Smash's identity as Tatsuya. He then lied to Sento about Tatsuya volunteering to become a Smash, and as he insulted Tatsuya by calling him a fool, Sento became angry and transformed into Build RabbitTank and attacked Stalk. Stalk and Build matched blows until Build gained the upper hand and performed an even more powerful Vortex Finish Rider kick, injuring Stalk's left hand in the process. Examining the damage to his arm, Stalk deduced that Build's anger has risen his Hazard Level to 3.7, confirming his theory. To Build's surprise, Stalk told him to save Tatsuya while he still can before leaving.

After learning that Ryuga knocked out Sento and readministered the Smash element into Tatsuya, whom then returned to Faust's lair, Stalk informed Rogue of Build's Hazard Level increasing, but Rogue was more concern of Stalk's plan. As Rogue asked him what he has in store, Stalk said he hungers for more strength, and that Build must learn more about himself before proceeding to the next stage. In the morning, Isurugi opened up nascita and blew a kiss at the sun, then proceeded inside to see Ryuga awake, telling him wha he learned from Misora last night about his failed attempt to transform with Sento's Build Driver. Grabbing Ryuga by the shoulder, Isurugi warned him not to do it again due to the danger it possesses before Ryuga shrugged him off. Sitting down in a chair, Ryuga asked where Sento was. Isurugi told him Sento went back to the Institute to find Faust's lair so he can rescue Tatsuya, which made Ryuga despondent at the idea of Sento trying to save a friend he can't remember. Isurugi told Ryuga that's not why Sento's trying to save Tatsuya; Isurugi revealed that Sento is the kind of person willing to help anyone in trouble. As Ryuga mocked Sento for being transcendent, but Isurugi told him Sento's just insecure for having no memory of his life and not know who he is, therefore he acts as he is now. He even goes further to say that deep inside Sento is nothing but loneliness that has no end. As Ryuga ponder on Isurugi's words, he drank a cup of coffee Isurugi made for him, shouting it's bitter as he swallows the awful brew. At night, Isurugi received a call from Gentoku, whom informed Isurugi of giving the bait with Katsuragi's data on the Nebula gas generated from the Skywall to Sento, expecting him to find a blindspot to allow him to infiltrate Faust's lair. Gentoku stated that Sento could even surpass Isurugi in intellect, and stated he's looking forward to seeing what happens next.

The next day as Sento and Ryuga infiltrated Faust's lair, Isurugi, as Blood Stalk, shoots one of Faust's human captives to prevent him from leaving and shouting at the other captives to be silent. Ryuga turns to Stalk, lunging at him with a fist as Stalk referred to him as a precious sample. Stalk easily locked Ryuga's arm in his own, allowing him to inject Ryuga with poison, placing Ryuga in agony as dark spots form under his skin. As Senro run to Ryuga's side, Stalk made a joke about injecting Ryuga with poison, but shrugged it off and stated he still has Sento. Confused Sento asked Stalk what he meant, and Stalk revealed to Sento that he was injected with a large dosage of the Nebula gas, but didn't transform into a Smash, like Ryuga, and it's because of the Smash element coursing within him he's able to transform into a Kamen Rider. With revelation and fury swelling within him, Sento transforms into Build RabbitTank and viciously attacked Stalk, swinging his Drill Crusher all over as Stalk dodged the attacks, destroying parts of the lair in the process. Forcing Stalk in a corner, Build tossed his weapon aside and dragged Stalk to the ground, pummeling him as he demanded to know whom injected him with the gas and erased his memory. As Stalk laughed while Build continues to pummel and demanded to have his memory restored, he told Build he loses perspective when it comes to his lost memory and shot him with his Transteam gun, forcing Build off and cancelling his transformation. He deemed that quality disappointing and decided to end their encounter by firing his gun at the ceiling, compromising the lair. With everyone in danger, Stalk thanked Sento for their brief encounter and left, laughing as Faust Guardians prepared to take the human captives with them.

After viewing the lair's destruction, Gentoku called Stalk, who was waltzing around in their backup lair. Stalk told him Sento now knows of his origin in the lab, leading to Gentoku wondering how long they intend to let Sento run free. Stalk assured him they will end him in time, but for now the next phase of their plan will become more interesting before tossing the phone away, ending their call. After returning to nascita at night, Isurugi sat in his chair as Sawa watched Ryuga gazing at Sento constantly. From Ryuga, they learned that Sento, as Taro a year ago, went to Katsuragi's apartment for a job interview involving new medicine testing. As Sento repeatedly told Ryuga he has no memory of killing Katsuragi and blaming Ryuga for the murder while Ryuga stand by that Sento's the murderer, all four people hear the booth go off. Sento rushed to see the new Panda bottle and walked down, believing that no one else but Ryuga believed he's the true murderer, but Isurugi and Misora played around and pretended to believe he did. Offering a suggestion, Isurugi told both Sento and Ryuga they should investigate into Katsuragi's life.

Returning from the Touto Institute, Sento came up with an anagram to decipher a letter Katsuragi left behind, deducing Katsuragi left his hidden data with his mother, Kyoka Katsuragi. Placing two cups of coffee on a tray, Isurugi proceeded to hand them to Sento, Ryuga and Misora as he listened to Sento's plan to find Kyoka, but to his shock Sento and Ryuga have coffee in cans. He walked toward the back in sorrow, a ploy as he heard from Sawa that Sento and Ryuga can enter into Hokuto with a Skyroad, one of the gaps in the Skywall leading Touto into Hokuto. However, she stated that SKyroad is governed by a person who won't let people pass without payment. Sento turned to Isurugi, who told him he can't help them as he has a part-time job and can't make decent coffee. With no other choice, Misora used her Miitan persona to acquire the money needed to use Skyroad as Isurugi slumped in a corner beside her.

Returning to Faust's backup lair as Blood Stalk, Isurugi summoned for Gentoku and Utsumi to return, informing Gentoku of Sento's plan to head to Hokuto, telling Gentoku it could disrupt his ultimate scheme: to utilize the Kamen Rider System as a military weapon, and stated if Sento gets caught by the Hokuto government. Gentoku decides to head to the Skyroad and stop Sento. He demands Stalk to accompany him, but he declines, forcing Gentoku to turn into Night Rogue and briefly engages Stalk before subduing him. Stalk concedes and agrees to accompany Rogue.

At the Skyroad passage between Touto and Hokuto, Stalk watched from afar as Rogue and two Faust Guardians intercept a family from Hokuto and the criminal overseeing the passage, calling Sento out as he threatened his captives. With no choice, Sento transformed into Build RabbitTank and duels Rogue, while Ryuga takes on the Faust Guardians, allowing the family and criminal to escape. After Rogue pushed Build ina corner and turned to Ryuga, Build brushed him aside with his Machine Builder, forcing Rogue to call on Stalk as Build and Ryuga headed for the passage. Forming his Steam Rifle and inserted the Rocket bottle in the weapon, Stalk performed a Steam Attack, but made it rebound towards Rogue, allowing Stalk to slip away into Hokuto unnoticed.

"As thanks for amusing me, let me tell you something...about Takumi Katsuragi. Katsuragi is the creator of the Smash. Takumi Katsuragi...created Faust!"

- Blood Stalk revealing the Smash and Faust's creator to Build and Ryuga.

Watching Sento and Ryuga talking to Kyoka as three of her students left, Stalk sneak-attacked the children, forcing Sento, Ryuga and Kyoka find Stalk standing over Takehiko, one of the students. Stalk told them he learned of their plan to find information on Katsuragi and told them to play with him as unleashed a Devil Steam with his Steam Blade, turning Takehiko into an Ice Smash. Ryuga asked how Stalk turned the kind into a Smash, and Stalk revealed his blade can administer Nebula gas into a human without Faust's equipment, angering Sento as he transformed into Build NinninComic, attacking Stalk and the Smash. Build used the 4Koma Ninpoutou's One Frame-Clone technique to split into two, leaving him to fight Stalk as his copy used the Four Frame-Steal technique to bring the remaining two kids to Ryuga and Kyoka before engaging the Ice Smash. After defeating the Smash and telling Ryuga to extract the element from Takehiko, the clone Build assisted Build as he fight Stalk until the clone disappeared, leaving Build to fight Stalk alone as Ryuga and Kyoka took the children to safety.

As Build turned into his PandaGatling Trial Form, Stalk combined his Steal Blade with his Transteam Gun, then proceeded to place the Rocket bottle in the Steam Rilfe and executed a Steam Attack, forcing Build out of his transformation and the Gatling bottle out of the Driver. As Stalk proceed toward the Gatling bottle, Ryuga attempted to engage Stalk but is easily caste aside, but the Cross-Z Dragon reached the Gatling bottle first and placed it in its back, allowing it to fire pellets of blue fire at Stalk. Ryuga took the chance to fight Stalk, but instead ended up removing the Rocket bottle from the rifle in a scuffle, and proceed to give it to Sento. Transformed into his new RocketPanda Best Match, Build overpowerd Stalk and performed a Vortex Finish Rider slash, forcing Stalk in a corner. Stalk, however, became amused as Build's strength increased once more, and left a parting gift to Build and Ryuga: Katsuragi is the creator of the Smash and Faust, and questioned their next move as he left via typhoportation.

Back at Faust's backup lair, Stalk was approached by Gentoku, whom returned from a meeting between the three Prime Ministers, and demanded to know why he made such a mess in Hokuto. Stalk assured him it can lead them to the thing they've been searching for years: Katsuragi's research data, and with it, they can upgrade the Rider System to a new level. The next day, Stalk observed from a distance as Sento and Ryuga found the location of Katsuragi's research data: at the abandoned Namba Heavy Industries Integrated Sciences Laboratory. Seeing Sento and Ryuga pulling out a flash drive from a buried container, Stalk revealed himself, stating he'd never thought Kyoka would hide her son's research in an abandoned building a he took the flash drive and tossed Sento and Ryuga aside. As Stalk thanked them, Ryuga attempted to fight him, but Stalk pushed him aside, forcing Sento to transform into Build HawkGatling and engage Stalk. From a nearby tree, Gentoku transformed into Night Rogue and joined the fight, engagin Build as Ryuga lunged toward Stalk again. Rogue told Stalk to let him fight Build, which Stalk obliged as he attempts to leave, but Ryuga engaged him once more. Catching Ryuga's fist, Stalk notice his Hazard Level has risen to 2.4, deciding to fight him for a while. As they fight, Stalk measured Ryuga's Hazard Level rising to 2.5, then 2.6 and 2.7, relishing in how it continues to rise. Tossing Ryuga aside, Stalk decided to hand him the driver, stating he wish to see how Ryuga progress before intervening Rogue's battle with Build, allowing him and Ryuga to escape. Rogue pointed his Transteam Gun at Stlak, demanding to know Stalk's scheme, to which Stalk assured him they don't need the drive, just the tools the data can create as he believes both Sento and Ryuga can do it.

Faust Invades Touto Institute
"Did you lose your principles as a scientist?" "Don't get the wrong idea. The Rider System was designed to be a military weapon to spill gallons of blood! Principles as a scientist? That's just your ego talking. I'm sure you understand...that science leads to destruction. As science progresses, humanity regresses...The environment is destroyed and the world perishes!"

- Stalk stating the harsh reality of science to Sento.

Watching Katsuragi's introduction video on the Build Driver and Kamen Rider Build form with Sento and Ryuga, Isurugi attempted to act hip with slangs as he commended the video. Ryuga asked him why he's talking like that, and Isurugi told him a girl in his part-time job was teaching him before reminding Sento he stole the Build Driver, the Fullbottles and the Pandora Box panel they had from Faust. He also added he tried to use the driver to transform, but nothing happened, and Sento told him only a person with a Hazard Level over the base level to transform and explained to Isurugi the different levels of people with Nebula gas resistance: Hazard Level 1 where the person injected with the element die soon afterward, Hazard Level 2 leads to the creation of the Smash, and people with a level over 2 are those who retain their human forms, to which Isurugi realized both Sento and Ryuga are at that point. Ryuga then pointed out that Stalk measured his Hazard Level as it rose during their previous encounter, leading Sento to conclude that a person with a Hazard Level 3 can transform into a Kamen Rider. Isurugi asked Sento if Katsuragi said anything else in the video, but Sento stated that Katsuragi was looking for a person who fit all the requirements to become Build through his human experiments with the Nebula gas. Isurugi then took notice of Sento and Ryuga arguing over the Smash experiments, leading to many casualties. As Ryuga blamed Katsuragi for seeing people as nothing more than guinea pigs and for the death of Kasumi, while Sento believed the science in the experiments were not at fault, Isurugi took his cue to leave.

After figuring out Ryuga took Misora sightseeing outside nascita, Isurugi, as Blood Stalk, informed Faust, sending Faust Guardians to their current location. Stalk arrived in between Ryuga and Night Rogue with Misora passed out, telling Rogue to not act behind his back. As both Faust members star at each other, Ryuga picked up Misora and fled, Rogue attempted to pursue but Stalk stopped him, unaware that Rogue had Faust Guardians corner Ryuga, only for them to be destroyed by Build NinninComic. Returning back to the lair, Stalk demanded to know why Rogue plotted behind his back, forcing Rogue to tell him that Juzaburo Namba, president of Namba Heavy Industries Ltd. and the benefactor of Faust, gave the ultimatum of giving him the Rider System or he will ally himself with either Seito or Hokuto. Cancelling his transformation, Gentoku stated he must keep the Rider System from being distributed to other countries as well, leading to Stalk asking him what he'll do. Gentoku then decided to take the Pandora Box from the Touto Institute, hoping it will cause a spark between the three regions to declare war.

The next day, Stalk waited patiently with a platoon of Faust Guardians in the the Touto Institute at the garage level. While Shingo Kuwata distract Build, Eita Kawai disengaged the security lock to the garage level, then proceeded forward in the Institute. Making his way to the entrance floor, Stalk injected four security officers with a poison that disintegrates their bodies, killing them instantly. Turning to a security camera over his right shoulder, he signaled Kawai to trap the three Prime Ministers, Utsumi, two security officers and the Pandora Box in an energy dome until he reached the eleventh floor where the box was placed on display. Along the way, he was shot at from behind by Touto Guardians and Touto Special forces, annoying him to snap his fingers and convert the Touto Guardians into Faust Guardians. As he reached the eleventh floor, the dome dispersed, allowing Stalk to proceed toward the Pandora Box. He killed the two security officers with his poison and commented on all the Prime Minsters being together before telling them he'll take the Pandora Box. Tajimi, Hokuto's Prime Minister, attempted to defy Stalk, whom pointed his Transteam Gun at her before firing at the security cameras.

Taking the Pandora Box, Stalk made his way to the roof until Ryuga, disguised as a Touto Security officer, approached behind him. Stalk commended Ryuga for being gutsy as the Faust Guardians engaged Ryuga, whom defeated all of them with the power of the Dragon bottle. Intrigued by Ryuga's power, Stalk attempted to fight him until Kawai appeared, telling Stalk to leave Ryuga to him. Obliging Kawai, Stalk infused him with the Devil Steam from his Steam Blade, turning Kawai into a Press Smash Hazard and leaving Ryuga to fight the Smash. On the rooftop, he heard Gentoku's voice from behind, turning to see him. As Gentoku held his Transteam Gun, he told Stalk the roof wasn't in their escape route. Stalk attempted to lie about getting lost, but Gentoku saw through it as he noted Stalk's scheme to shoot out the security cameras to cover his tracks. Transforming into Night Rogue, he asked Stalk to handover the box, to which Stalk declined, annoying Rogue and forced him to engage Stalk. Ryuga, with a few cuts and bruises, interrupted their fight. Stalk asked him what happened to the Smash, and Ryuga stated he beat it but couldn't extract the Smash element without an empty Bottle. Rogue was shocked Ryuga managed to defeat a Smash without a Rider System, while Stalk became amazed.

Ryuga then engaged Stalk, only to be quickly defeated as Ryuga's fight with the Smash left him scathed until Build HawkGatling arrives to assist him, forcing Rogue to assist Stalk. Rogue fights Build while Stalk engaged Ryuga once again. During the fight, Stalk told Ryuga his attacks won't hurt him until they started to cause damage. After Ryuga punched his left arm, Stalk realized Ryuga's Hazard Level has risen to 3.0, praising him for finally awakening his full power. However, it wasn't enough as he knocked Ryuga down, which was caught by Build FireHedgehog, whom ceased his attack on Rogue and attacked Stalk with fire. While Rogue sneak-attacked Build, Stalk inserted his Cobra Fullbottle in his Steam Rifle and executed a Steam Shot at both Rogue and Build, knocking Rogue to a wall and not only cancelling Build's transformation but force him to drop the majority of his Fullbottle collection as well. As Rogue recovers, Stalk takes the Pandora Box and leaves, angering Rogue.

On ground floor again, Stalk noticed the defeated Press Smash Hazard approaching him. Surprised it can still move, Stalk inserted his Cobra bottle into his Transteam Gun, executing a Cobra Steam Break shot, finishing the Smash long enough for Stalk to extract the Smash element from it, reverting it back to Kawai. As he walked away, Kawai asked him if he was Katsuragi, which Sento heard as he caught up to Stalk. KAwai thought Katsuragi faked his death because he was planning something huge, but Stalk assured him Katsuragi is dead. Refusing to believe that Kawai stated Katsuragi could've used modern technology to transfigured a corpse. Intrigued, Stalk walked over and demonstrated such ability, by painfully transforming Kawai's face into Kuwata's. As Sento rushed to an unconscious Kawai, Stalk leaped into a Faust Guardians' Union State, turning it red and fleed, with Sento, transforming into Build RabbitTank and on his Machine Builder, pursuing him. Stalk attempted to lose Build by shooting at him, but Build managed to destroy one leg with the Machine Builder's front wheel forming a spiked energy wheel, then taking heavy damage from Build's Drill Crusher gun mode before being finished by the Machine Builder's axle saw, forcing Stalk out of the gestalt machine.

In pain from the explosion, Stalk turned to Sento, whom believed Stalk was Katsuragi, and was asked why he faked his death, framed Ryuga and injected Sento with the Nebula gas. Stalk asked Sento if that's what he shoulder really be asking, forcing Sento to ask him why he created Faust, the Smash, and if the Rider System was a tool to protect the innocent. Disgusted by Sento's words, Stalk pushed him down and told him the Rider System was a military weapon for bloodshed and scoffed at Sento's claim of a scientist's principles and added science leads to destruction, which is evident by humanity's actions on the world with the environment being destroyed. Sento doesn't believe that and said if used properly, science can bring people happiness, but Stalk asked him if he was sure, and reminded him of the Nebula gas experiments harming lives until Rogue knocked Sento out from behind. Rogue asked Stalk where he planned to take the Pandora Box as a dozen Faust Guardians surround them, their rifles pointing at Stalk. Stalk told him to their lair as he offered the box. Rogue then took the box, and grabbed Stalk, gesturing to him to not try a stunt like that again, and stole the Rabbit, Tank and Lion bottle from an unconscious Sento. Stalk watched as Sento came to and grabbed Rogue's leg, demanding his bottles back before Rogue kicks him in the face, then proceeded to walk away laughing.

Rise of Cross-Z & Cover Exposed
"I can't do it...I can't possibly do it! You're the one who made me who I am now! It's thanks to you that I learned how to be human. I believed you...You saved me...But...There's no way I could kill you!" "Guess the battle's over then."

- Sento reveals he's incapable of killing Stalk, whom is Isurugi.

Despite showing Gentoku he was still with Faust, Blood Stalk was not informed of where the Pandora Box was placed, forcing him to infiltrate the Touto capital, marching in on Gentoku and Utsumi after a meeting with Tajimi and Mido. Utsumi attempted to halt Stalk, but he pushed him aside, continuing to walk toward Gentoku, whom told Stalk to leave. Stalk made a comment about Gentoku's temporary position as acting Prime Minister has made him cold and asked why Gentoku placed the box in a secure location without informing Stalk. Gentoku told him if he plays nice, he reveal the location, which irritated Stalk as two Touto officers barged in the room, preparing to arrest Stalk. Stalk, however, killed them both with his disintegration poison before telling Gentoku Faust doesn't belong to him.

Finding out Gentoku set a trap for Sento and Ryuga to take the Dragon and Lock bottles, Stalk went to the abandoned warehouse in time to see Gentoku, as Night Rogue, stand a few feet away from Ryuga and a defeated Sento. Stalk praised Rogue for acting behind his back, but Rogue was upset Stalk found out about his plan. Ryuga argued with Sento as he took his Dragon bottle back in an attempt to transform and expose Stalk's identity, but Stalk assured him he came to help them, but Ryuga doesn't believe Stalk, saying he's the one who frame him and vowed to defeat as he attempted to place the Dragon bottle in the Cross-Z Dragon, but failed. Annoyed, Stalk told Sento to take Ryuga and leave, throwing a canister at Rogue who attempted to shoot them with his Transteam Gun. As Rogue asked Stalk why he stopped him, Stalk scoffed and left, leaving an angry Rogue walking out of the warehouse.

After Ryuga transformed into Kamen Rider Cross-Z and defeated the Octopus Smash, whom was revealed to be Sawa, Isurugi was with Misora, Sento and Ryuga as Sawa confessed she was a spy for Faust and Namba Heavy Industries. She also revealed that she already knew about the secret lab's entrance and grabbed a matchbox from the bowl to back up her lie that Sento dropped a mathcbox the day he rescued her. As both Sento and Isurugi celebrated that they didn't leave any tell-tales of the lab, Misora cut them off and told Sawa to continue, and Sawa revealed her job was to get close to them and pass along any information to Namba's company. Isurugi then explained to everyone that on the outside, Namba's company is the top industrial leader, but in reality they manufacture weapons, smuggle goods and other illegal dealings. He even adds that the three Prime Ministers can't go against Namba before Sawa revealed that Namba is a benefactor for Faust. Ryuga doesn't believe Sawa's claim, and Sento added if Namba was Faust's benefactor, then Faust knew about nascita being a front for Kamen Rider Build from the get-go and asked Sawa why Faust didn't attack them. Sawa stated they didn't attack nascita because they wanted to gather data on Build so Namba can use the Rider System for warfare. Sawa then claimed she was acting as Faust's spy to gain information on them, which she backed up with a chip in her necklace, which is a recording file of a conversation between Rogue and Namba while she was abducted before her transformation into the Octopus Smash. In the recording, it mentioned the location of the Pandora Box, which leaves everyone puzzled on where the location is. Isurugi stated it can't be back at the Touto Institute, and Sento wondered where the box was before it was placed in the Institute, which Sawa revealed it was at Namba Heavy Industries Integrated Science Laboratory before the lab was abandoned.

Isurugi, as Blood Stalk, walked in on Night Rogue, Kamen Rider Cross-Z and Sento, finally figuring out the box was placed in the third basement in the abandoned laboratory. Rogue demanded to know why Stalk came, to which Stalk replied, to Rogue's surprise, to fight him. As they fought, Stalk spotted a newly paved wall and thrusts his Steam Blade in it to create a hole, exposing not only the Pandora Box, but the Fullbottles Rogue stole from Sento, the new Densya bottle and Faust's Pandora Box panel. Rogue pushed Stalk away from the box and bottles, allowing Sento enough time to retrieve his bottles and transform with an additional Build Driver into Build KaizokuRessya. As Build intervened in their fight, Cross-Z forces Stalk down the hall as Rogue and Build engage one another. Cross-Z claimed to Stalk he'll unmask him as they fought, forcing Stalk on the ground from Cross-Z's Beat Closer Million Hit slash. Rising, Stalk praised for Cross-Z and decided to leave with the Pandora Box via pyroportation.

Returning to nascita at night after making a deal with Namba, Isurugi went into his act and became surprised and happy to see all of Sento's Fullbottles and the second green panel in the lab. Isurugi praised Ryuga and Sento, but he was confused to see Sento building something, which Misora revealed it to be new weapon, which was inspired by his new Best Match: Kaizoku Hassyar. Finished, Sento swung his new weapon at Isurugi, who ducked down screaming frantically, then at Misora, Ryuga and Sawa before attempting to test the weapon's capability on Isurugi, who ran behind a beam as he told Sento not to point it at him as Ryuga grabbed Sento. The next day as Namba's contigency to destroy Faust was commenced, and Utsumi was a scapegoat for Gentoku as Night Rogue and as the creator of Faust, Stalk saved Utsumi after Gentoku shot him off a bridge, dragging him to land from a river.

At night, Isurugi returned to nascita, finding Ryuga, Sawa and Misora confused as Sento was left alone in the secret lab. Isurugi asked Sento if he was alright through the fridge door, completely unaware that Sento was upset after learning from Nabeshima, whom recovered his memory, that Isurugi is Blood Stalk. Attempting to drawn Sento out, Isurugi made grilled horse mackerel and fanned the odor, which drew Sento from the lab with a smile on his face. As Sento leaned against a column, Isurugi asked him if Nabeshima really called him, to which Sento confirmed and revealed that Nabeshima also regained his memory. As Ryuga asked Sento if Nabeshima remembered who framed Ryuga, Sento stated Nabeshima was just following orders from Stalk, but unaware to Isurugi, Ryuga, Misora and Sawa, Sento concocted a plan to confirm if Isurugi was Stalk.

The next day, Isurugi received a summons from Gentoku and arrived as Stalk, meeting Gentoku in the woods. As Stalk pointed out it's not like Gentoku to call for a meeting, he demanded the location of the Pandora Box from Stalk, which Gentoku learned that Namba was cutting him out of their deal with the Rider System the previous night, but Stalk joked about Gentoku being straight to the point. Angering Gentoku, Stalk decided to reveal the location, but in return he wants Gentoku to swear loyalty to him, which infuriated Gentoku even more to transform into Night Rogue and settle the score with Stalk once and for all. Stalk asked Rogue if thinks he can win against him as Rogue mentioned the Transteam System, unlike the Rider System, keeps the user's Hazard Level at a fixed level, making them both equal as Rogue made the first move. Rogue gained the upper hand with his bat abilities, Steam Blade and Transteam Gum, forcing stalk to the ground until he decided to let experience determine the outcome as he slipped under Rogue via typhoportation and summoned his Steam Rifle, overpowering Rogue with a combination of assaults and blasts, forcing Rogue out of his transformation. As he pointed his rifle at Gentoku, Stalk decided to reveal that he made a deal with Namba and became his new partner, meaning the box is located at Namba Heavy Industries. He then pointed the predicament Gentoku is now in if he utilizes the Touto government to secure the box, which will lead to Namba siding with Seito or Hokuto and declaring war. As he walked away, Stalk mentioned if he was in Gentoku's predicament, he should use someone unconnected to the Touto government, hinting Sento.

Returning to nascita at sunset, Isurugi spotted Sento sitting by the front counter, asking him what's wrong. Sento changed the subject by welcoming Isurugi back and asked him if he'd just returned from his part-time job, to which Isurugi confirmed and said he'll be heading back there tomorrow. Sento made a joke about which of Isurugi's job is real, and Isurugi stated he's just trying to keep nascita open, even when no customer has come for a cup of coffee. Isurugi revealed that nascita is a special place and asked Sento if he remembered the day Isurugi found him and became Kamen Rider Build. Isurugi stated from that day, nascita was Sento's home, and he admitted he'd like to welcome Sento back after his fights with the Smash, making him, Sento and Misora a family.

As Isurugi expected, Sento and Ryuga broke in the 7th Touto plant of the Namba Heavy Industries to infiltrate the massive storage facility, which Gentoku informed Sento that the facility is where the Pandora Box was located. Waiting in a screening room with the box, Isurugi, as Blood Stalk, tampered with the door, allowing it to open as Sento and Ryuga, as Kamen Riders Build KaizokuDensya and Cross-Z, failed to open the door. As both riders entered the room, Stalk informed them the door's password changes every three hours, which is something Sawa should've noticed during her reconnaissance of the Touto plant. Build asked if Stalk let them in to tell them that, but Stalk shrugged it off, saying he doesn't need to thank him and admits he takes pleasure in toying with them. An angered Cross-Z made the first move as Build hesitated before assisting him against Stalk. To the riders' surprise, Stalk was able to counter their every moves, deflecting Build's Kaizoku Hassyar's shots with his Steam Blade and send him sliding aside as he engaged Cross-Z. Build transformed into HawkGatling and rejoined the fight, adding aerial assaults as Cross-Z continues his assaults, but Stalk grabbed him and used him as a shield against Build's HawkGatlinger's shots. Spotting an opening as Build hovered over Cross-Z, Stalk formed his Steam Rifle and shot Build down, who then transformed into Build NinninComic and used his 4Koma Ninpouto's duplication jutsu, gaining the upper hand until Stalk inserted his Cobra bottle into the rifle, executing a Steam Shot that took out the Build duplicates before knocking Build down.

After demonstrating his strength, Stalk told Build whatever tactics and attacks with each Best Match is futile for Stalk knows all of them. As Build rose, Stalk was confused to see a new bottle: Octopus, and with the Light bottle, Build transformed into a new Best Match form: OctopusLight. Build utilized the BLD Light Bulb Shoulder and Shade Breaker, blinding Stalk as he attack him with his Twist Lash Arm, overpowering Stalk before tossing him across the room. As Cross-Z rejoined the battle, both he and Build executed finishing attacks: Build with a Vortex Break Rider punch and Cross-Z with a Lock powered Mega Slash from his Beat Closer, bring Stalk down to the ground. Cancelling his transformation, Sento revealed to Stalk that he wouldn't be able to anticipate Sento's attack with a new bottle. Stalk admitted he was surprised to see a new bottle, but then he pointed out all of Touto's Fullbottles have been purified, which was Stalk's goal all along as he prepared to leave. Before he could, Cross-Z attempted to stop him, forcing Stalk to unleash a energy cobra from his symbol, which distracted the riders long enough for Stalk to leave via typhoportation, and leaving the Pandora Box to Sento and Ryuga.

At night, Isurugi returned to nascita, heading down the lab to see the Pandora Box in the middle of the lab. Pretending to be surprised, Isurugi commended both Sento and Ryuga for their work until he spotted them, with Misora, eating a large banquet with the money from the cash register, desolating Isurugi. Ryuga stated Isurugi wasn't using the money, forcing Isurugi to join in their feast. As Misora and Ryuga slept, Isurugi took the eighteen of the twenty bottles, both green panels and the Pandora Box, existing the lab and proceeding out the front door until Sento stopped him, and both realized the jig is up. Sento revealed that Nabeshima remember Isurugi's true identity as Stalk, but Isurugi demanded proof as he pointed out that Nabeshima could be lying. Sento also agreed, but he came up with a plan to confirm Nabeshima's claim: by having Misora purify the Octopus bottle in secret, which also revealed to Sento that the reason Stalk was always aware of Sento's plan was because he knew about them. Isurugi commended Sento for his ingenuity, but Sento wanted to hear from Isurugi's words that he is Blood Stalk. Turing to Sento before smiling, Isurugi bursted out of nascita, forcing Sento to pursue him.

Isurugi ran a long distance with Sento on his trail until he collapsed near a bridge, panting from exhaustion as Sento caught up to him. As Isurugi made a claim about getting old, Sento asked him why he did this, make Sento into Kamen Rider Build to defeat the Smash when Isurugi was part of Faust the whole time, which Isurugi revealed it was all part of a larger scheme he had. He jokingly asked if Sento would let him go, but he knew Sento wouldn't. Accepting it, Isurugi also pointed out he still need to get the Octopus and Light bottles, which Sento realized Isurugi lead him to the bridge away from pedestrians. Claiming to go all out, Isurugi pulled out a Transteam Gun and the Cobra bottle, which confirmed he is Blood Stalk, and transformed. Stating he is Stalk, Isurugi activated the suit's voice modulator since he only spoke through it when he was Stalk. Sento transformed into Build OctopusLight and attacked Stalk with the Twist Lash Arm,which was blocked by Stalk's Steam Blade. Pushing Build back, Stalk made a comment about the Octopus bottle being the most annoying due to his hatred for octopi as Build executed a Vortex Break Rider punch, driving Stalk back. As Build ran toward him, Stalk pulled the Pandora Box from his pouch and cast a blinding light, followed by energy, at Build, which cancelled his transformation and knocked the Octopus and Light bottles out of his Build Driver. Retrieving the last two bottles, Isurugi cancelled his transformation, satisfied he now has all the Touto bottles, the green panels and the box, bidding farewell to Sento as he left, completely unaware that Sento switched all the twenty bottles with fakes and that one of the empty bottles, that fell from Sento's jacket, had absorbed the energy from the box.

The next day, Isurugi presented the box, green panels and twenty bottles to Namba, informing him all the Touto bottles purified and asked he wants to do with the box when it's opened. Namba revealed that he wants the energy within the box to create a weapon surpassing a nuclear bomb, thus solidifying Namba Heavy Industries' position as the number one weapons manufacturer, which Isurugi found scary. As Namba placed the Rabbit bottle in one of the green panels, it didn't light up but played a recording of Sento's voice saying "Mismatch", which was followed by the remaining nineteen doing the same thing, shocking both Namba and Isurugi as they realied they're fake. Isurugi grabbed the fake Gorilla bottle, which ejected a blue flag with "Too Bad" in japanese kanji; Isurugi commended Sento for being ahead of him this time. Later on in an abandoned storage facility, he called Sento, stating he need to settle things with him before taking his plan to the next stage, which Sento agreed and both decided to meet in the alley Sento, as an amnesiac Katsuragi, met Isurugi for the first time.

While it rained, Isurugi, with the Pandora Box, waited in the alley until Sento arrived, claiming this scene is nostalgic as he placed the box down. Sento asked Isurugi if the entire time he spent with Sento, Ryuga and Misora was all a lie, to which Isurugi said that some of their time together wasn't a lie, commenting on how Sento's development made Isurugi feel bad for deceiving him. Infuriated, Sento tossed his umbrella aside and put on his Build Driver, inserting the Rabbit and Tank bottles in the belt. Isurugi obliged Sento by tossing his umbrella aside and pulled out his Transteam Gun, inserting the Cobra bottle in it and transformed at the same time Sento did. Both Build and Stalk stared at each other before lunging toward one another, their punches casting a shockwave for a moment before they begin to fight. Being stronger, Stalk forced Build into a construction facility, trading blows with Build until he gained the upper hand. As Stalk prepared another punch, Build countered with a Vortex Break attack from his Drill Crusher with the Ninjya bottle inserted in it, driving Stalk back until he drew his Steam Blade and continued his attacks. Both fighters continue to trade blows until Build stopped, due to his memories with Isurugi, allowing Stalk to savagely assault him with his Steam Blade and Transteam Gun, forcing Build out of his transformation as Sento fell to the ground. As Stalk shook his head in dissatisfaction, Sento, filled with sorrow, confessed he can't kill Stalk as he was the one who made Sento the man he is now after his amnesia. Stalk, however, showed no similar emotion and stated, with his voice modulator off, that the battle's over as he pointed his gun at Sento until it was knocked out of his hand by Ryuga's Beat Closer.

Turning to see Ryuga, whom overheard the conversation, decided to fight in Sento's place and transformed into Cross-Z, fighting Stalk as he stated his anger doesn't come from his framing by Stalk, but of how Stalk manipulated both Sento and Misora. As Cross-Z continued his assaults, Stalk retrieved his gun and inserted his Cobra bottle in it, executing a Steam Break shot, sending Cross-Z back and cancelling his transformation. Before Sento rushed to Ryuga's side, Stalk asked Sento why he didn't find it strange that he rescued Misora from Faust, and asked Sento to become Build to fight the Smash. To Sento's shock, Stalk revealed that it was all to make Misora purify the Fullbottles by making her believe it was for justice as she refused to purify them after learning Faust was using them for malice purposes, even when threatened she still refused. Stalk then approached an angered Sento that his role as Kamen Rider was just to help Misora purify the bottles for justice, while in reality Sento was a puppet as he grabbed Sento by his jacket before tossing him back down. As Stalk revealed how far his manipulation ran, he also added to both Sento and Ryuga they can't defeat him. Stalk then turn to Sento, whom accepted to being deceived but that both him and Ryuga weren't puppets, they fought to protect others and grow their strength, and to protect people. Sensing Sento's new resolve, Stalk paid no attention until he spotted a new item Sento pulled out, the RabbitTank Sparkling bottle, which Sento placed in his Build Driver and transformed into a new form: Build RabbitTank Sparkling.

Leaping forward with heightened speed, Build delivered a powerful kick to Stalk, forcing him back. Stalk attempted to counter with a few punches, but Build blocked them and attacked Stalk with stronger blows. Stalk tried using his Steam Blade, but Build knocked it from his hands and continued with power counters, forcing Stalk to a beam. Commending Build's new power, Stalk, with his voice modulator turned on, summoned two energy cobras from his symbol, but Build was stronger than them and forced them out through the roof. Heading outside, Stalk was shocked to see how strong Build became, whom then executed a Sparkling Finish Rider kick, destroying both cobras and sending Stalk, whom attempted to block the attack, flying back to the alley as he transformation came undone. Isurugi, struggling to rise, couldn't believe he was beaten by Build, whom arrived and cancelled his transformation, severing all ties to Isurugi. Isurugi commended Sento for his actions and added that if Sento was able to build the RabbitTank Sparkling bottle, then he can complete a new device in Katsuragi's research data. Before leaving, Isurugi revealed to both Sento and Ryuga that Gentoku is Night Rogue and decided to leave the green panels and the Pandora Box with Sento, stating that he looks forward to see how both riders will progress in future events as he bid them farewell and leapt out of the alley.

Hokuto Declares War on Touto
to be added

The Forbidden Item & Siding with Seito
Handing the Hazard Trigger, the "forbidden item" of Project Build, to Sento, Isurugi told him that it may grant the power he needs to stop Ryuga from going berserk at the cost of Sento himself losing his sense of self, revealing that the Hokuto Three Crows who used it beforehand are condemned to die upon defeat due to being Hard Smashes. Before taking his leave, Isurugi explained that, with Ryuga and Sento respectfully representing nature and nurture, his intent is to push them to both become stronger for a goal which he would reveal later.

After the 3-on-3 Proxy Battle ended with victory for Touto, Blood Stalk was summoned by Namba to kill Seito's Prime Minister Masakuni Mido when he refused to dishonor his promise to relinquish his claim to the Fullbottles and the Pandora Box. Blood Stalk then used his power to modify Namba's face into that of the late Prime Minister, allowing him to personally take control of Seito and launch his own invasion of Touto using the full arsenal of Namba Heavy Industries Ltd..

Personality
1="Why didn't you finish him off... Stalk?" "I'm the game maker. I derive the best strategy by considering every possibility. Everything is going as planned."

- Blood Stalk to Night Rogue

2="Have you lost your mind?" "I told you before. Faust isn't yours!"

- Blood Stalk's disregard for Night Rogue's authority

3="With Kamen Rider Build defeating evil monsters...Misora was able to purify the Bottles, thinking she was doing it for justice. In other words, that was all you existed for. You were merely acting the hero of justice. You were just playing Kamen Rider."

- Blood Stalk unveiling his manipulative persona to Sento

4="So you've finally awakened!" "I am Rogue...Kamen Rider...Rogue...."

- Blood Stalk noticeing Night Rogue's awakening as Kamen Rider Rogue

Ten years ago, Soichi was said to be a kind and caring person. However, upon his first exposure to the Pandora Box's energy, Soichi was transformed into a ruthless, cunning mastermind.

Soichi's persona as nascita's owner is an enthusiastic but laid back man who paternal demeanor served as a gentle guidance to Build's team. While he appears to have some genuine sentiment for this makeshift family, Soichi made it clear that he sees all of them as means to an end.

As Blood Stalk, he is a self-proclaimed strategist. He also seems to have a lot more of an unperturbed and more evil attitude compared to the more focused Gentoku, nonchalantly fooling around even when doing his job as a Faust enforcer. Such is seen when instead of directly engaging Build, he was casually laying back and letting the Square Smash do most of the fighting, until it was the right time for him to interfere by making the Smash grow. He also dislikes taking orders from anyone, preferring to do things his own way.

Soichi also viewed his battles with both Sento and Banjou as an experiment to measure their Hazard Level. He was visibly entertained when both of them had a rise in their Levels and gave them hints in return. He isn't even bothered by opposing Gentoku temporarily to let them escape so he can see how strong they become.

Soichi can be cruel and ruthless if he wants to, making him not much different from Gentoku in regards to their aggression. He is more than willing to commit more heinous acts, such as killing several Touto guards using his venom, and changing Eita Kawai's face into that of Shingo Kuwata just to show he could. Like Gentoku, he views science and the Rider System as instruments for war.

Soichi has a tendency to sprinkle bits of Italian in his speech, most commonly saying "Ciao" as a goodbye.

It is later revealed that Soichi's old personality still exists within him, as his old personality was actually suppressed by another entity much like Vernage possessing his daughter, Misora.

Powers and Abilities

 * Computer Proficiency: Isurugi has some level of programming knowledge, as he was able to hack the GPS on a stolen motorcycle, and steal the completed Sclash System data from Sento's computer.
 * Altered Physiology: Being the first human to be exposed to the Pandora Box, Soichi has demonstrated numerous superhuman abilities since his return from Mars. Unbeknownst to everyone, including Soichi himself, these powers came from the entity that possessed Soichi.
 * Hazard Level 3+: As a user of Transteam Gun, it seems that Soichi has a high Hazard Level that allows him to use such device. As Stalk, Soichi mentions that his and Gentoku's Hazard Level are at the same level and would stay the same due to the Transteam System being incapable of rising them.
 * Matter Reshaping: Isurugi was able to manipulate a tablet computer into a rod shaped item that he used as a back scratcher.
 * Superhuman Agility: Isurugi was shown to be able to leap at a scale far beyond human capacity after his battle with Build's RabbitTank Sparkling Form.
 * Isurugi switching face.pngIdentity Manipulation: Using a cloud of steam from his hand, he can make anyone's face look like anyone else's; the process seems painful to those who are awake to feel its effects. While the permanence of the change is yet unclear, certain individuals have been shown to retain their new identities for more than a year. Initially thought to be an ability of the Blood Stalk system, this power has since been revealed to be a power gained from Soichi's time on Mars. However, it was shown that this power could be applied only if the copied person is dead or killed. This process also seems to have an effect on the subject's physiology as well, as seen when Juzaburo Namba discarded his walking stick and no longer needing glasses to see upon having his appearance changed into that of the dead Masakuni Mido. This process has also been demonstrated to be reversible at Soichi's discretion.
 * Isurugi memories wipe out.pngMemory Manipulation: Soichi has shown the ability to erase the memories of others with nothing more than a touch of his hand. Unlike the amnesia induced by Nebula Gas, Soichi's memory manipulation is shown to be complete, causing a total wipe of the person's previous identity. However, with time, some victims have shown to be able to recover partial memories, skills, or even the entirety of their past given proper circumstances.
 * Pyrokinesis: Soichi could shoot a crimson flare from his hand or with his Transteam Gun at the enemy. Its power is enough to destroy Banjou's Dragon Sclashjelly and Sclash Driver. He is able to use this power in both his civilian form and as Blood Stalk.
 * Pandora Box Control: Soichi could manipulate Pandora Box power to do his bidding, either to attack enemy with various phenomenons or to change Pandora Tower inner structure.
 * Dagger Proficiency: Although a very subtle example, Soichi is capable of throwing knives at a high precision rate.

Devices

 * Transteam Gun - Transformation device and personal weapon.
 * Fullbottles - Transformation trinkets.

Weapons

 * Steam Blade - Standard sidearm weapon. Can combine with the Transteam Gun to form the Steam Rifle.

Portrayal
Soichi is portrayed by Yasuyuki Maekawa (前川 泰之). As Blood Stalk, his modified voice was voiced by Tetsuo Kanao (金尾 哲夫), who previously voiced Namakelder in Happiness Charge Pretty Cure!, which would later be Soichi's other voice as a human. As Blood Stalk, his suit actor is Kazuya Okada (岡田 和也).

Etymology
The katakana "sutāku" (スターク) in his name is usually translated as "Stark".