Asa Mizushima is a high school sophomore who transferred to East School from a neighboring city. Seeking emotional refuge after her parents' divorce, she joined the school's swimming club shortly after arriving. She stands 163 cm tall and weighs 50 kg, with measurements of 88-55-86 cm. Her appearance features long, straight black hair reaching her chest, typically styled with blunt bangs, and eyes depicted as either gray or brown. She is commonly seen in her school uniform, a swimsuit, or sportswear, sometimes accessorized with a neckerchief or bag.

Outwardly blunt and brusque, Asa masks underlying shyness, particularly around new people, which complicates her social integration. Despite her rough exterior, she exhibits girlish vulnerabilities and internal conflicts. During summer remedial classes, she encounters Hajime Saisho, a confident swim club member. He persistently pursues her with romantic and sexual advances, initially using photography as a pretext. Though resistant at first, Asa gradually becomes involved with him both physically and emotionally.

Their relationship grows complicated by Natsume Sorayama, a gardening club member who also develops feelings for Hajime. This creates a tense dynamic where Asa experiences significant guilt, shame, and discomfort regarding the physical aspects of her involvement with Hajime. Eventually, Asa and Natsume agree to "share" Hajime for the remainder of the summer. This resolution intensifies Asa's emotional turmoil as she vacillates between resisting and succumbing to her desires. A time jump reveals her reunion with Hajime and Natsume years later at a train station, indicating lingering connections from their summer arrangement. Her narrative arc centers on navigating conflicting emotions, interpersonal challenges, and personal vulnerability within this unconventional relationship.