Funetaro Momoki manages the Wellside division of the Kura organization, directing operations of the Mizuhanome System which analyzes serial killers' unconscious minds, known as id wells. He possesses grey eyes and short, spiked dark blue hair featuring a prominent cowlick. His attire consistently includes a black suit, grey waistcoat, off-purple shirt, and grey tie.

Momoki balances stern professionalism with profound empathy and a dedication to saving lives. He holds a firm belief in his duty as a police officer to protect others. This conviction drives him to save Kiki Asukai—the final victim of the serial killer called the Challenger—refusing to let her die or commit suicide while promising her eventual salvation. Despite his disciplined demeanor, he occasionally indulges curiosity, such as continuing to observe a closed id well to analyze its emotional symbolism. He maintains an amicable relationship with his imprisoned friend Akihito Narihisago, offering encouragement after id well dives, a contrast to others who shun Narihisago for his past.

His background includes a close professional history with Narihisago, having worked alongside him as a police officer before Narihisago's imprisonment. Momoki was initially designated as successor to Chief Takuhiko Hayaseura but chose to continue as Wellside Director under Shirou Kokufu after Hayaseura's departure. A personal investment in Kiki Asukai's case compels him to secretly investigate her disappearance for years, suspecting deeper mysteries linked to the enigmatic figure John Walker.

During the series, Momoki faces false arrest under suspicion of being John Walker—a setup orchestrated by Hayaseura to conceal his own identity as the true antagonist. Following Hayaseura's exposure and defeat, Momoki resumes his role as Wellside Director, now reporting to Chief Kokufu. His commitment to resolving Kiki's plight persists, culminating in efforts to safely extract her from the Mizuhanome System after the crisis.