Mamiya belongs to the Freemums, an organization protecting "stray" Minimum Holders who manifested abilities outside Facultas Academy. He works security at Club Without, enforcing the members-only policy and maintaining order.

Tall and thin, he wears round glasses and secures his hair with a headband. His initial outfit consists of a black v-neck shirt, grey pants, a long green sweater coat, and black shoes, accented by a silver neck chain and ear piercings. His appearance later evolves to a suit and tie with rectangle-shaped glasses, and eventually a police uniform.

He maintains a level-headed, calm, and serious demeanor, rarely smiling yet not appearing cold-hearted. Mamiya operates cautiously, particularly voicing skepticism about the Freemums' alliance with Art. His loyalty centers on Ishigami Shunichi, working closely with him to secure freedom for stray Minimum Holders.

His interactions with Birthday involve conflict; he initially attempts to deny Birthday and Ratio entry to Club Without, escorting them to meet Ishigami only after Ratio criticizes the venue. In a later encounter, Birthday threatens him during another access denial. Mamiya's initial skepticism towards Art shifts dramatically after witnessing Art's resurrection via Regeneration Minimum, leading him to regard Art highly alongside other Freemums.

Mamiya wields the Decadence Minimum, inducing frenzied overindulgence in individuals consuming food he has contaminated with his power; its activation method remains unspecified. He also possesses advanced technological skills, demonstrated by hacking a Minimum Holder jail's air vent system and using his phone to broadcast combat footage to Club Without attendees.

No concrete background details exist across any official media, including the *Fw:Hamatora* compilation film. His development remains consistent through *Re:_Hamatora*, with no additional information provided in spin-offs like *Mini Hama*.