Yoichi Terai serves as a patrol officer at the Kouen-mae Police Box, sporting a chubby, middle-aged build with curly dark brown hair and circular glasses that hide his eyes in manga depictions, though the anime reveals them. His standard uniform—a blue suit, white undershirt, blue pants, black tie, and shoes—shifts to short sleeves during summer episodes.

Originally Yoichi Terai, his identity gains a whimsical twist in the manga from Volume 170 when a children’s name-generating toy renames him Yangukan Marui, a change the anime omits to retain his original name. This shift coincides with fleeting financial luck, though property or vehicle purchases often spiral into mishaps. A Hokkaido native, he balances married life with two sons, a stable familial backdrop seldom detailed in the series.

Terai’s neutral, pragmatic demeanor grounds the police box’s eccentric dynamics. He approaches situations with realism, shuns extremes, and indulges in routine interactions like snack-sharing or casual chats. Yet pressure exposes his timid side, leading him to acquiesce to peers’ outlandish requests. Beyond duty, he cultivates oil painting, calligraphy, and encyclopedic knowledge of Japan’s rail networks from daily train commutes.

As a mediator, he frequently counters Kankichi Ryotsu’s antics but occasionally falters due to hesitancy. His rapport with Keiichi Nakagawa features casual honorifics, signaling mutual respect, while Daijiro Ohara trusts his diligence in tracking Ryotsu’s movements.

The character’s backstory remains largely unexplored, with no arcs delving into personal growth. His birthday ambiguously fluctuates between March 2 and May 5 in the manga, though the anime anchors it definitively on May 5, aligning with Japan’s Boys’ Festival. Terai endures as a steadying, relatable figure amid the series’ comedic turbulence.