Tetsuo Tetsujin, one half of the "Iron Brothers" alongside his twin brother Tetsuro, shares a background as former laboratory guinea pigs where they believe they developed psychic abilities. These claimed abilities, including flight or traversing time and space, only function while they wear their distinctive iron helmets; the brothers assert the helmets prevent them from floating away and shield them from military detection, yet also inhibit their active use of the powers. Their community under the bridge has not verified these claims.

The brothers hold a position within the riverside community, working under Stella and managing the communal hot baths housed in repurposed oil barrels. They exhibit jealousy regarding the relationship between Kou Ichinomiya and Nino, perceiving Kou as an outsider disrupting their social dynamics. This jealousy occasionally drives them to interfere with the couple.

Identified by matching sailor suits and metal helmets, Tetsuo and Tetsuro function as a largely inseparable unit, with Tetsuro noted as the older twin. Their personalities blend loyalty to the community structure with a mischievous streak, often targeting Kou but generally avoiding serious malice. While their past laboratory experiences are referenced, specific details about their lives before joining the community or the nature of the experiments remain unelaborated. Their development primarily involves integration into the community's routines and ongoing, mostly comedic interactions with Kou and Nino, without significant individual evolution detailed in the available material.