Seibetsu Fumei na Chuusei Bokukko to Oneesan

Hinako, a 26-year old office worker, is confused by Itsuki's, her neighbor, gender, whom she thought was a boy when they were a child. Hinako spends her daily life surprised by Itsuki's casual gestures. However, it is actually Itsuki's heart who is pounding the most, having unrequited feelings for Hinako.

(Source: Kadokawa, translated)