Maiko Osonoi is a cheerful and energetic teenage student, a schoolmate and friend of Nana Sunohara. She has short ochre hair reaching her neck and petrol blue eyes, standing 157 cm tall. Her given name translates to "dance child" (舞子), while her surname means "small garden well" (小薗井).

Maiko delights in playful and mischievous antics, often alongside her friend Mea Uchifuji. Both share an obsessive attraction and perverted interest in younger men, particularly directed at the main character Aki Shiina. Maiko demonstrates self-awareness regarding legal boundaries, explicitly questioning Mea's ambition to become a kindergarten teacher over concerns about criminal intentions. She rationalizes her own interest by aspiring to become a lifeguard, aiming to legally admire half-naked boys at pools.

Her role centers on comedic interactions, primarily appearing with Nana and Mea. She participates in teasing or doting on Aki but maintains a secondary presence compared to the primary residents of Sunohara-sou. Her traits and narrative function remain consistent as a supporting character contributing to lighthearted or fan-service-oriented scenarios.