Ayumu Hoshino is the main character, Kazuma Hoshino's, little brother.
At the very start, Kazuma and his little brother Ayumu move to the Yamabiko countryside. This is because Ayumu gets ill easily, and their parents sent them to Yamabiko to see if the countryside would do Ayumu any good. If it did, they would join their sons later.
While waiting at a bus stop, a monkey steals Ayumu's hat, so Kazuma runs after it into the forest. However, he ends up lost and eventually runs into Ui Natkasugawa, who shows him how to get back to the bus stop. When it comes to crossing the river, Kazuma slips and falls into Ui, and accidentally kisses her. Ui's close friend Ibuki Hinata saw this, and is always yelling at Kazuma for some reason or another.
After arriving at the Yorozuyo Inn, the place they'll be staying, Ayumu and Kazuma meet Tsumusi Toudou, who works part-time at the inn. Tsumugi is Kazuma's guide for school as well, and her little sister Kasane looks after Ayumu.
Ayumu is a slightly shy character, easily flustered. He's quite artistic and is in the art club at his junior high school. Ayumu and his older brother Kazuma were moved to the countryside by their parents to see if it would do Ayumu, who gets ill very easily, any good. If the plan was successful, the Hoshino parents would join them after a few months.
Ayumu's condition does get better, and his confidence grows as well.
In the anime, Ayumu Hoshino is played by Ohashi Ayuru
Ayumu has flat mid-long hair, like his brother, but unlike his brother, Ayumu's hair is a very light brown. He has indigo eyes like Kazuma and is usually seen in his school uniform or a shirt and trousers/shorts.
Kazuma Hoshino: Ayumu's older brother. He is very protective of Ayumu.
Ayumu does not have any known relationships, but he does state in the first episode after his first day at his new school that he really likes one of his new friends, 'The Ears of Yamabiko', Kasane Toudou.
Ui Nakatsugawa : Ui is Ayumu's brother Kazuma's friend from school. Ayumu invites Ui over to Yorozuyo, the inn he is staying at, to draw her as a present for Kazuma, as he thought Ui and Kazuma were going out.
Kasane Toudou: Kasane is Kazuma's guide Tsumugi's little sister. Kasane looks after Ayumu on the first day of his new junior high, just as Tsumugi watches out for Kazuma.
Tsumugi Toudou Kazuma's first-day-of-school guide. She also works part-time at the Yorozuyo Inn. Tsumugi tell Ayumu that to thank his brother for everything he's done for him, he should just try to make him smile, once for every time Kazuma made Ayumu smile.
Madoka Koumoto: A very shy girl who works at Yamabiko's shrine. When Kazuma was little he visited Yorozuyo Inn and played with Madoka, though he doesn't recognise her because of her shyness and long hairstyle.
Ibuki Hinata: Ui and Kazuma's friend from senior high school.
Koyori Toudou: Tsumugi and Kasane's outgoing and strong-willed little sister.
Hina Sakai: Hina is Tsumugi's friend from school. She thinks that Ayumu is very cute, and is disappointed when Ayumu can't make a trip the group trip due to his art club.
Daigo Minamikokubaru: Daigo is Ayumu's brother Kazuma's school friend. He insists on calling Ayumu 'bro' because 'My bro's bro is my bro too, bro!'
Senka Yorozu: The manager of Yorozuyo Inn.
Ayumu - Kazuma
Ayumu-san - Ui, Tsumugi etc
Bro - Daigo
Hoshino's little brother - Hina