Hina Tachibana (橘 陽菜 Tachibana Hina) is the sister of Rui Tachibana, step sister and teacher of Natsuo Fujii. Hina is also the ex-girlfriend of Shū Hagiwara. Hina is also one of the main heroines of the series, as she appears in the first chapter of the series.
In the colored pages of the manga of Domestic Ka Kanojo, Hina is a beautiful girl with a dark shade of brown hair and light green eyes. Hina's age is never explicitly given but it is said that she is new to being a teacher, so it is implied that she is in her mid to late 20's. This is further enforced by Kine in chapter one, who says that they aren't 'brats' to Hina as "(Hina) is only 23".
Hina is a gentle woman who is conflicted on her feelings of love. Hina fell deeply in love with Shuu, so much so that it caused a family issue between her, Fuji Natsuo, and Rui Tachibana. However after letting him go, she developed feelings for Fuji Natsuo.