Terai Youichi is a police officer assigned to the Kameari Park police box in Katsushika Ward. Plump and always wearing glasses, he aspires above all to live as an ordinary policeman, seeking a conventional and stable life. Yet persistent financial difficulties plague him, compounded by his limited intelligence, though he maintains a sensible disposition. His attempts at major purchases, like a house or car, consistently end in misfortune.

He is frequently seen working alongside his superior, Chief Ohara. Around 2009, the character's name changed from Yōichi Terai to Marui Yangukan, a shift that coincided with an improvement in his fortunes. As a father of two sons, Terai shoulders family responsibilities. Within the narrative, his ongoing financial struggles and earnest pursuit of normalcy serve as a grounded counterpoint to the more eccentric personalities at the police box.