Toshio Ohtomo is a childhood friend of Yuu Morisawa and Midori Kisaragi. He often displays a dismissive attitude toward Yuu, teasing or belittling her, masking his genuine underlying care. This dynamic fuels frequent comedic conflicts, with Yuu retaliating physically or verbally against his remarks.

Midori Kisaragi, Toshio's best friend, harbors romantic feelings for Yuu and repeatedly enlists Toshio's help in confessing or pursuing her, creating a minor love triangle. Toshio remains largely oblivious to Yuu's own romantic interest in him during their early relationship.

A significant shift occurs when Toshio witnesses Yuu's accidental transformation into her idol persona, Creamy Mami, around Christmas Eve. This discovery directly causes Yuu to lose her magical abilities. Toshio then accompanies Yuu to Feather Star, where the alien Pino Pino offers to restore her powers only if Toshio's memory of her magical identity is erased. Toshio consents to the memory alteration to help Yuu.

Despite the erasure, Toshio later finds a recording he made of the transformation incident, triggering the return of his suppressed memories. This leads to his ultimate realization that Creamy Mami and Yuu are the same person. By the series' conclusion, he acknowledges that his affection for the idol was fundamentally an extension of his true feelings for Yuu, culminating in a mutual understanding.

Outside these relationships, Toshio functions as Creamy Mami's most devoted fanboy. He avidly attends her concerts and events, collects merchandise, and openly expresses his admiration. This idol worship creates internal conflict for Yuu, who desires his attention in her real identity. His character arc resolves with the reconciliation of these dual aspects of his affection.