Kai Deguchi operates as co-leader of the Musashino Saru gang, wielding a signature baseball bat. His past includes a deep high school friendship with Mera, shattered when their rival gang affiliations collided. This rupture stems from Mera killing Kim and two Musashino Saru members after misidentifying them as Shibuya Saru rivals, igniting the central conflict.

Kai's development begins as he hunts for his friends' killer, culminating in a confrontation where Mera kills Tera, the pacifistic Musashino Saru leader. This thrusts Kai into reluctant leadership, forcing him to navigate escalating gang warfare. He wrestles with vengeance while striving to uphold the Saru’s founding ethos: providing sanctuary from inter-gang violence against threats like the Bukuro Wu-Ronz and Shinjuku Hands.

Kai sustains key bonds with Saru members Hashim and "Secretary" (Hajime Tamiya), embodying the gang’s "Band of Brothers" dynamic centered on mutual protection and peacekeeping. His interactions with Sunmi, a prostitute linked to antagonist Buppa’s organization, introduce complexity as she becomes entangled in his feud with Mera.

Though occasionally noted, Kai’s background as a DJ receives less focus than his gang leadership and combat roles. His arc consistently revolves around managing violence’s fallout and pursuing justice for fallen comrades amid expanding conflicts involving multiple Tokyo gangs and external threats like the Neo Wu-Ronz/Skunk Empire.

The live-action adaptation preserves Kai’s core identity as the Musashino Saru’s central figure but shifts emphasis to Mera as a primary antagonist seeking citywide territorial control. This version condenses and alters specific relationship dynamics and plot points from the source material.