Mephisto emerges as an enigmatic figure within Demon City Shinjuku, wielding considerable power and extensive knowledge of the district's supernatural forces despite undefined origins. He operates with ambiguous motives, initially presenting as a detached observer rather than an active participant against Rebi Ra.

During the core conflict, he provides crucial sanctuary and medical aid to injured protagonists. His decisive interventions include eliminating a pursuing demon and freeing a key character from ritual sacrifice. While his actions align against the demonic incursion, he maintains an air of distance from human affairs. Following the immediate threat's resolution, he implies potential future encounters.

Beyond these events, expanded media reveals his role as director of a Shinjuku private clinic, earning the moniker "Demon Doctor" for profound expertise in medicine, chemistry, history, technology, and magic. Rumors persist of an alleged ability to resurrect the dead, which he denies. His primary combat method employs specialized metal wires, mirroring Setsura Aki's fighting style, supplemented by traps and alchemical concoctions.

His personal history includes a noted aversion to women, complicated by a past romance with a vampire. He forms a recurring investigative partnership with Setsura Aki, a Shinjuku private investigator specializing in missing persons cases. Their collaboration confronts supernatural threats, including a vampire group led by "Princess" aiming to transform Shinjuku into a vampire domain. He maintains professional connections like information broker Ryoko Sototani and the witch Galeen Nuvenberg, who perished protecting Setsura.

Mephisto's demeanor remains typically aloof, analytical, and strategically minded. He exhibits confidence in his capabilities, adopts a pragmatic approach, and while capable of cooperation, sustains an emotional detachment from others.