Ayumu Hoshino is Kazuma Hoshino's younger brother. Possessing a slender build and a somewhat feminine appearance, he has wavy blonde or light brown hair and indigo eyes. His fragile physical health, stemming primarily from chronic asthma, drives the brothers' relocation from the city to the rural town of Yamabiko, undertaken for the benefit of his respiratory condition. Their parents may join them if his health improves.

Enrolling in a local junior high school, Ayumu joins the art club, reflecting his primary hobby of drawing. He displays friendliness, altruism, and approachability, though he is also shy and easily flustered. His innocent and trusting nature leads him to comply with unconventional requests, such as wearing a cheerleader uniform to support Kazuma. His confidence gradually increases as his health stabilizes in the countryside.

His relationship with Kazuma is central, defined by Kazuma's protectiveness and Ayumu's deep appreciation for his brother's care. Ayumu desires to reciprocate Kazuma's kindness, guided by the advice to "make him smile" for every instance of encouragement received. Among peers, he forms a notable friendship with Kasane Toudou, who assists him during his transition to the new school and whom he openly admires. He is also close to others, including Ui Nakatsugawa, whom he invites to model for a drawing intended as a gift for Kazuma. Kazuma's friend, Daigo Minamikokubaru, affectionately refers to Ayumu as "bro".

Ayumu lives at the Yorozuyo Inn under the care of Senka Yorozu, a family friend. His role emphasizes interactions within this community and his gradual adaptation to rural life, contributing to his personal growth and improved well-being.