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 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.

History
Ten years prior to the events of Build, Isurugi 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 rushed the security guards and applied his hand to the Pandora Box, 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 in Touto.

Seven years later, he was appointed as a special consultant for the Pandora Box, while also covertly joining Faust in the process. Around the same 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 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.

Sometime later, he confronted Takumi Katsuragi about him trying to quit Faust. At that time, Taro Satou had just arrived for his drug-testing job and was killed by Stalk. Knocking Katsuragi out, Stalk switches the two's faces and clothing, and leaves Taro Satou (with Takumi's appearance) there. He takes the out-cold Katsuragi (with Satou's appearance) and erased his memories before talking him back to Faust to experiment on him.

On September 5th, he brought the now-amnesiac Katsuragi to a small alleyway on a rainy day. While waiting for Katsuragi to wake up, he went to buy an umbrella before coming back to a now awake Katsuragi. Soichi renames him Sento Kiryu and nurtured him into Kamen Rider Build. This was to ensure that as Build continues fighting more monsters, he'll find needs to purify more bottles by having Misora do so.

Isurugi, as Blood Stalk, made contact with Sento shortly after Sento defeated Masahiro Nabeshima as the Mirage Smash by stabbing him in the back with a venomous tentacle, leaving him stunned. Stalk then used a giant projection of a cobra to retrieve the incapacitated Nabeshima. Gentoku Himuro seems to be aware of Blood Stalk's activity.



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 nuture, his intent is to push them to both become stronger for a goal which he would reveal later.

Personality
Ten years ago, Soichi was said to be a kind and caring person. However, upon 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.

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

As Blood Stalk, he is a self-proclaimed strategist. He also seems to have a lot more of an unperturbed 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.

Human

 * 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 SclashSystem 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.
 * 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.
 * 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.
 * 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.
 * Dagger Proficiency: Although a very subtle example, Soichi is capable of throwing knives at a high precision rate.

Blood Stalk

 * Venom Whips: On each wrist, he has extendable venomous tendrils that can shoot out and stab victims, injecting them with venom capable of either paralyzing, killing, or even disintegrating them.
 * Activation Command: As Guardians are in fact sleeper agents of Faust, Stalk can snap his fingers to activate their true programming into serving him.
 * Typhoportation: He can escape any situation by becoming a cloud of red smoke and flowing away.
 * Energy Cobra Projection: Through his suit, Stalk can create an energy cobra projection that is used to attack or kidnapping victims or even to make quick escape from enemy attack.
 * Guardian Merging: Blood Stalk can merge with multiple Guardians becoming a Union State Form to turn it into a red variant and pilot it himself.
 * Voice Changer: As mentioned by Takumi himself, Stalk's equipment has the ability to change its user's voice tone freely to hide their identity.
 * Enhanced Durability: The true purpose of the Blood Stalk system is to encourage the rise of Hazard Levels in the Nebula Gas victims, thus he is shown capable of accepting far greater punishments from either Build or Ryuga. Instead of showing pain or discomfort, Stalk is otherwise enjoying the beatings so long that his opponent's Hazard Level rises.
 * Hazard Level Measuring: Blood Stalk could measure current hazard level of Kamen Riders / Smashes by simply touching them

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!. 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".