Ryuga Banjo

Ryuga Banjou (万丈 龍我) is a 23-year-old ex-fighter who was wrongly imprisoned for the murder of a scientist named Takumi Katsuragi, who was involved with the Pandora Box. Abducted by Faust while in prison, Ryuga managed to escape with his humanity intact, which would lead him to join Sento Kiryu in uncovering the truth behind their shared experiences as Faust victims.

He would later use the Cross Dragon given to him by Sento to become Kamen Rider Cross-Z (仮面ライダークローズ), the secondary rider of Kamen Rider Build.

Character History
Ryuga was born in Yokohama, weighing 3.2 kg. As an adult, Ryuga became a professional fighter, eventually getting engaged to Kasumi Ogura. By the age of 22, Ryuga earned a lifelong ban from fighting for throwing a match; it is implied that he accepted a bribe in order to pay for the frail Kasumi's medical bills. To help his search for a new job, Kasumi directed Ryuga to Takumi Katsuragi, a researcher at the Touto Institute of Advanced Matter Physics, who was apparently looking for an assistant. However, when Ryuga came to Katsuragi's home to apply, he found the latter's corpse inside. Moments later, Touto's Guardians arrived and apprehended Ryuga. Framed for Katsuragi's murder, Ryuga was sentenced to ten years in prison.

While in prison, he was attacked by a prison guard working for Faust, who began experimenting on him to turn him into a Smash. Somehow, Ryuga managed to escape mid-experiment, retaining his memories and humanity, and retreated into the sewers.

Meeting Sento Kiryu
Ryuga was eventually found by Touto's Guardians, who attempted to recapture him. Ryuga managed to fend off the Guardians and escaped before he was apprehended by Sento Kiryu, who had tracked him upon noticing his Smash-like energy readings. Upon realizing that Ryuga had undergone a similar experience, Sento began interrogating Ryuga in hopes of uncovering his past, despite Ryuga's frustrated belligerence. The two were interrupted by Strong Smash, who attacked Ryuga. However, Ryuga was saved by Sento, who transformed into Kamen Rider Build and defeated the Smash. Ryuga recognized the Smash's human identity as one of his fellow victims, but was frustrated to find that the man had no memories of the experiments. The Touto government arrived soon after to retake Ryuga. Sento, believing in Ryuga's innocence, helped him escape.

Burn Smash incident
Sento brought Ryuga to the basement lab of the nascita café, where the former had been living for the last year. After meeting Sento's accomplices, Souichi and Misora Isurugi, Ryuga explained the cirumstances that resulted in his arrest. However, the story was interrupted by Sawa Takigawa, a freelance journalist who had entered the base (due to Souichi and Sento's carelessness).

As a precaution, Ryuga was chained up in the lab, much to his chagrin. Misora took the time to explain the Fullbottle purification process to Ryuga, as well as Sento's job. Noticing Misora's smartphone, Ryuga goaded her into proving her ability to purify Smash energy. As planned, Misora left her smartphone where Ryuga could reach it, allowing him to contact Kasumi. However, much to his horror, Kasumi had been kidnapped by Faust as well. Given Kasumi's location by Night Rogue, Ryuga broke out of nascita and raced to save his fiancé. After another encounter with Guardians (which ended with being saved again by Build), Ryuga explained his motives to Sento, but choked him into unconsciousness to prevent him from getting involved.

When Ryuga finally arrived at the location, he learned that he was too late: Kasumi had already been transformed into the Burn Smash. Unable to see his beloved hurt, Ryuga begged Sento not to attack her when the latter arrived. The two were quickly overwhelmed by the Smash and Night Rogue, upon which the latter revealed the horrifying truth: Kasumi's body was too weak to be restored, and removing her Smash essense would be fatal. Realizing that Kasumi could not be saved, Ryuga begged Build to end her suffering. Sento was able to separate Kasumi's consciousness long enough for her and Ryuga to say their goodbyes. Left almost broken by grief, Ryuga would have let himself be captured again if not for Sento, who reminded him that it would all be for nothing if he got arrested again. Kasumi's Smash essence was purified into the Dragon Fullbottle, which Sento gave to Ryuga as a memento of his beloved. Soon after, Sawa arrived, having discovered the identity of the guard who attacked Ryuga: Masahiro Nabeshima.

Continuing the Search
After some searching, Ryuga managed to contact Nabeshima. Nabeshima revealed that his family was being threatened by Faust, upon which Ryuga offered to smuggle them out of Seito to safety in exchange for Nabeshima testifying for his innocence (not realizing this was a trap set up by Night Rogue). With Sawa's help, Ryuga snuck to Seito. Upon retrieving Nabeshima's wife and daughter, Ryuga and Sawa were attacked by hijacked Guardians, barely escaping with their lives. Ryuga arrived in Touto to aid Build in taking down Nabeshima, who had been subjected to two Smash transformations by Faust. While Nabeshima was returned to his human form, the experiments wiped his memory clean, leaving him unable to recognize his daughter, let alone testify for Ryuga.

The next lead came in the form of Tatsuya Kishida, who was roommates (as well as bandmates) with Sento before the latter was captured.

Personality
Ryuga is a hot-blooded and impulsive man, tending to lash out quickly. While nowhere near as intelligent as Sento, Ryuga prefers to confront problems directly using his fists and doesn't want to involve other people in his own problems. Also, unlike Sento, he does not overthink things, preferring to use common sense, as shown when he was easily able to guess two Best Matches on his own. He is shown to easily get startled, a good example is when the bell goes off indicating a Fullbottle is ready.

Beneath his tough exterior is an honest, selfless and honorable individual. It is implied that the cheating that banned him from fighting was to get money for his fiancé's medical treatments.

Powers and Abilities

 * Hand-to-Hand Combat: Ryuga is a skilled fighter, having fought professionally before getting a lifelong ban.
 * Nebula Gas Compatible Physiology: Ryuga's physiology has been altered as a result of Faust's experiments, though he manage to retain his humanity upon being exposed to Nebula Gas, and the only one to experience no memory loss. According to Blood Stalk, these subjects are extremely more unique than Smashes. However, unlike Sento, Ryuga cannot utilize the Build Driver to transform.
 * Superhuman Strength: Ryuga has displayed enhanced strength: he was able to fight off numerous Guardians on his own, and break metal chains with his bare hands.
 * Fullbottle Augmentation: By using a Fullbottle in his hand without transforming, Ryuga can gain abilities based on said Fullbottle's traits:
 * Dragon: Ryuga's punches give off powerful shockwaves. Notably, his strong synchronization with the Dragon Fullbottle causes his power to increase with each use. According to Blood Stalk, his attacks have a Hazard Level of 2.4 and then increases to 2.7 the more he fights.
 * Smash Energy Signature: Ryuga gives off an energy signature similar to Smash, which alerted Sento to his location.

Arsenal

 * Drill Crusher: Build's personal weapon that Ryuga borrows to attack with.

Devices

 * Build Driver - Transformation device.
 * Cross Dragon - Slot adapter for Build Driver and personal familiar.
 * Fullbottles - Transformation trinkets.
 * Sclash Driver - Secondary transformation device.
 * Sclashjellies - Secondary transformation trinkets.
 * Build Phone - Smartphone that transforms into the Machine Builder.

Weapons

 * Beat Crosser - Cross-Z's personal sword.

Vehicles

 * Machine Builder - Rider Machine shared with Kamen Rider Build.

Portrayal
Ryuga is portrayed by Eiji Akaso (赤楚 衛二), who previously portrayed Hiroki Nagase in Kamen Rider Amazons Season 2.

Etymology
The kanji "Ryu" (龍) used to spell his first name means "Dragon".