Aki Shiina is a teenage boy with a feminine appearance, leading to frequent misgendering. He possesses chin-length black hair, light green eyes, fair skin, and stands 141cm tall. Lifelong misgendering, worsened by his overbearing older sister Matsuri Shiina dressing him as a girl, drove him to move to Tokyo. He sought independence to affirm his male identity and cultivate masculine traits, enrolling in a local middle school while living at the Sunohara Inn.

At the inn, he developed romantic feelings for Ayaka Sunohara, the maternal caretaker. Ayaka's affectionate and physically close behavior, including lap pillows, often confused him about her intentions. He remained determined to prove his maturity by helping with chores. His efforts to assert his masculinity faced complications from other female residents, especially student council members: Yuzu Yukimoto, aware of his gender and exhibiting a sibling-like rivalry; Sumire Yamanashi, who sees him as an older sibling figure; and Yuri Kazami, who pressures him into women's clothing due to his looks.

His kind-hearted nature and the residents' disregard for boundaries frequently placed him in awkward situations, such as accidental partial nudity or intimate contact. These incidents typically resulted only in Yuzu's tsundere outbursts rather than severe retaliation. Despite initial anxiety and shyness, he showed determination and passion in daily interactions.

As time passed, Aki experienced physical development. By manga chapter 52, he grew taller, surpassing Matsuri and nearing Ayaka's height. He adopted a shorter, neater hairstyle and wore more boyish clothing like hoodies and sweatpants. This growth reflected his ongoing journey toward self-assuredness, though his romantic pursuits and struggles with societal gender perceptions continued. His core relationships, including the complex dynamic with Matsuri and evolving bonds with the residents, remained central to his path of self-acceptance and identity.