RK, which stands for Red Knife, is an organized crime group based in Kamurocho featured as one of the primary antagonistic groups in Lost Judgment. They came to power following the fall of the Tojo Clan by exploiting the power vacuum left behind in their wake. In the Japanese version of the game, RK is described as a Hangure group, because while they are an organized crime group, they are not a yakuza clan as they do away with the codes of chivalry and honor that defined their yakuza predecessors.
RK was founded and is lead by Kazuki Soma, an undercover agent for the National Police Agency's Public Security Division and former executive candidate of the Tojo Clan's Nikkyo Consortium, shortly after the dissolution of the Tojo Clan in 2019.
Soma and the NPA's purpose for the gang was two-fold: keep control of Kamurocho's organized crime by centralizing it into a gang under police control, and provide a way to easily monitor the city's criminals. To guarantee the gang's success, Soma's handler in the NPA, Hidemi Bando, provided Soma with ample funds that he could use to entice former Tojo members to join RK by promising first-rate pay. The group was also given a Western-sounding name to distance it from yakuza traditionalism, something meant to attract the youngest former Tojo soldiers who'd become disillusioned with the yakuza after their clan collapsed. The name Red Knife itself was an allusion to Soma's weapon of choice when killing male victims.
By 2021, RK became the largest and most infamous gang in Kamurocho's criminal underworld. RK proper had only around a hundred members, whose membership was ranked on a tier-based system, from A-Tier to F-Tier, much like characters would be ranked in fighting video games but while regular goons start with F, the ex-Tojo start with B. Ranked members of the gang would be paid for every job they performed or racket they ran by getting a percentage of the job's earnings. On top of those hundred members, the gang also had several hundred more "freelancers", who would be brought in to work occasional jobs at a fixed rate.
Unlike the Tojo and Omi, RK have no moral scruples or honor in their operations, consisting mainly of the worst of the worst by even the Japanese underworld standards as their members casually resort to violence and enjoy a deal of oversight from the Public Security Bureau and considers civilians fair game.
Soma and his many minions, the foremost of which being the gang's second-in-command Daimu Akutsu, constantly cross paths with Takayuki Yagami and his friends, including Soma and Akutsu's former fellow Tojo Clan member Masaharu Kaito (whom Akutsu unsuccessfully attempts to have join RK). Soma's motivations are to apprehend Jin Kuwana due to how Bando is attempting to find a political weakness in Reiko Kusumoto for his own political purposes, and Kuwana assissted Kusumoto in carrying out a vigilante murder. To this end, one of their goals is searching for Shinya Kawai, a man who disappeared that is related to the murder.
Yagami's opposition to RK reaches a fever pitch after he finds that Soma has killed Yoko Sawa to silence her. Later in the game, when Akutsu fails to defeat Yagami's group, Soma has RK henchmen wearing police uniforms assassinate him. In the end, Yagami defeats Soma in a warehouse and he is arrested, causing the presumable end of the group.