Toshio Ōtomo is Yu Morisawa's childhood friend, around 13 years old at the series start and turning 14 by episode 47. He shares a close yet contentious bond with Yu, frequently bickering in a manner reminiscent of a married couple. Despite this, he genuinely cares for her well-being and acts protectively when she faces challenges. His initial attitude towards Yu is dismissive, partly because he perceives her as immature, a contrast to his notably sweeter younger self during their early childhood.

Toshio develops an intense romantic infatuation with Creamy Mami after her debut television performance, becoming one of her most devoted fans. He regularly attends her concerts and public events, often with his best friend Midori Kisaragi. This obsession creates conflict for Yu, who secretly harbors a crush on Toshio and becomes jealous of his admiration for her magical alter ego. His obliviousness to Yu's feelings and her dual identity fuels recurring comedic tension, often resulting in physical retaliation from Yu or Midori after his belittling remarks.

Midway through the series, in episode 25, Toshio accidentally witnesses Yu transforming into Creamy Mami, an event causing her to lose her magical abilities. Yu and Toshio then travel to Feather Star, where the alien Pino Pino offers to restore her powers if Toshio's memory of the transformation is erased. He agrees to this sacrifice. However, he later discovers a tape recording of the incident, triggering fragmented memories that gradually lead him to recall the truth.

By the series conclusion, Toshio fully comprehends that Creamy Mami and Yu are the same person. This realization culminates in his acknowledgment that his feelings were for Yu all along, transcending his infatuation with her idol persona. His character arc resolves with this acceptance and a deeper emotional understanding of their relationship.