Ryuga Banjo

Ryuga Banjou is the deuteragonist of Kamen Rider Build, having the power to transform into Kamen Rider Cross-Z. He was a former professional fighter who was disqualified for cheating. He was later framed for the murder of Takumi Katsuragi, a scientist studying the Pandora Box and was experimented on by Faust, but managed to escape and take refuge with Sento Kiryu, joining him in his fight to bring down the evil organization.

Past
Before Ryuga was born, his mother was exposed to the essence of Evolt while she was still a month pregnant, causing Ryuga to be born early.

As an adult, Ryuga became a professional fighter, but was banned from the sport after he threw a match so he could pay for his girlfriend Kasumi's medical bills. Forced to get a new job, Kasumi suggested Ryuga go to Takumi Katsuragi, a scientist who was supposedly looking for a lab assistant.

Framed for Murder
However, when Ryuga went inside Katsuragi's house, he found the corpse of Takumi Katsuragi (in actuality, the corpse of Taro Satou who had his face swapped with Katsuragi by Blood Stalk) inside. He was then apprehended by a squad of Touto Guardians for Katsuragi's murder.

Sentenced to ten years in prison for the murder, while in prison, Ryuga was attacked by Masahiro Nabeshima, a prison guard working for Faust, who injected him with Nebula Gas, but due to his unnaturally high Hazard Level, he managed to retain his humanity and escape.

Meeting Sento
While being chased by Touto's Guardians, Ryuga ran into Sento Kiryu, who was curious about how his energy readings resembled that of a Smash. The two were then attacked by Strong Smash, but Sento fended it off as Kamen Rider Build. When the Touto Guardians arrived to recapture Ryuga, Sento helped him escape, believing that he was innocent.