Hoshino was born on April 21, 1980, in Shiga Prefecture, Japan, as the younger of fraternal twin girls and the second of three children. As a child, she originally dreamed of becoming an astronaut. Hoshino confessed she was not too interested in school, often making excuses to avoid taking classes. Additionally, she added that would often practise drawing in the middle of the classes which gave her doubts when she had to erase them. She drew her first manga at twenty-one. As a child she used to read the manga Dragon Quest: Dai no Daibōken that her brother collected, as well as series by Fuyumi Soryo and
Makimura Satoru, which her mother bought. However, she instead prefer anime, most notably Studio Ghibli films. Her favorite film was Nausicaä of the Valley of the Wind (1984) to the point of watching it multiple times. When watching Castle in the Sky (1986), Hoshino decided she wanted to become an animator, inspired by director Hayao Miyazaki mostly. When going to highschool, Hoshino started drawing which she believed to be the result of reading Naoko Takeuchi's manga Sailor Moon. She often showed the 4 panel drawings known as yonkoma drawings to her sister who was in the same college. This 4komas often involved a monkey. Additionally, Hoshino stated she was a fun of the comedy genre as she aimed to make people laugh.
When Hoshino was 17, she aimed getting animation jobs against her parents' whishes but she failed. Next year, she tried doing it again and was accepted. Hoshino moved to Tokyo to become an animator. Despite becoming an artist, Hoshino found problems with her job. Due to financial issues, Hoshino was thinking about going back to her parents but her sister convinced to go to Kyoto. At that time, her sister motivated her to get a job but she failed most of them. At an unknown time, Hoshino was motivated by her mother to start writing manga. She still felt troubles in her new job, due to the multiple hours at work, the problems with storyboards and having to take the bullet train. Nevertheless, Hoshino succeded in her work. She dreamed of bringing her mother to the city and was able to in 2006. After graduating from high school, Hoshino did not know what career should she follow as she was offered the work of joining video game companies. Ultimately, she decided to become a manga author despite her fears of the difficulty it takes.