Ayumu Hoshino, the younger brother of Kazuma Hoshino, relocates with his brother to the rural town of Yamabiko ahead of their parents. This move, prompted by Ayumu's asthma and generally frail health, is intended to benefit his condition; their parents plan to follow if the countryside proves helpful. The brothers stay at the Yorozuyo Inn, run by family friend Senka Yorozu.

Ayumu possesses a slender build and a somewhat feminine appearance. His light brown or ochre hair is mid-length and flat, contrasting with his brother's style, and he has indigo or violet eyes. He typically wears his junior high school uniform or casual attire like shirts with trousers or shorts.

Initially shy and timid, especially around new people in Yamabiko, Ayumu often stays close to Kazuma for reassurance. Despite this, he is inherently friendly, kind-hearted, trusting, and innocent, making him susceptible to persuasion—like agreeing to wear a cheerleader uniform to support his brother. Artistic by nature, drawing is his primary hobby, and he joins his school's art club.

Ayumu's health improves in Yamabiko, and his confidence grows. He becomes more socially open, forming friendships at his junior high school. His gentle, polite nature and occasional feminine mannerisms make him popular among the town's girls, sometimes leading to him being "repurposed" in surprising ways. He remains unbothered by these situations despite perceived personality differences from Kazuma.

Ayumu shares a close bond with his highly protective brother, Kazuma. He admires Kasane Toudou, the younger sister of Kazuma's guide, Tsumugi Toudou; Kasane looks after him during his initial school days. Ayumu interacts warmly with other residents, including Ui Nakatsugawa, whom he invites to the inn to draw as a gift for Kazuma, mistakenly believing Ui and Kazuma are romantically involved. He also develops friendships within his school and the broader community, such as with Kazuma's friend Daigo Minamikokubaru, who affectionately calls him "bro".