At first, no official photo of Hoshino had been published early in her career; she was thought to be male until her first public appearance in Germany in 2008, especially since her editors referred to her as male. 

Hoshino claimed to have been teased about wearing a toupée in her childhood, because her first name Katsura is also a Japanese word meaning toupée, though it means Cercidiphyllum. The kanji for Katsura can also mean moon, leading Hoshino Katsura to mean "starry field beneath the moon".

Hoshino has a twin sister and a younger brother. To please her sister, and because she liked to draw, she used to draw manga every day. She drew her first manga at 21.

Hoshino's debut manga Continue appeared in the Weekly Shonen Jump in 2003. She also wrote an unpublished manga called Book-man. Many of her characters come from earlier works such as these while some are modeled after real people such as famous scientist sand, in one case, a silver accessory brand name. She is the author of the manga pilot Zone, which was published in Shueisha's magazine Akamaru Jump. This later paved the way for the D.Gray-man series, which began in 2004 and was serialized in Weekly Shonen Jump. An anime, as well as two novels, have been adapted from the series. An English dub of the anime was released in early 2009 from FUNimation.

In November 2008, D.Gray-man was put on a hiatus due to Hoshino falling seriously ill and once again because of an injured wrist. The continuation of the series was transferred to the monthly magazine Jump SQ.

Once an assistant of Takeshi Obata.