The manga Hajimete no Onii-chan centers on Kokoro, a young girl who was in a relationship with an older boy named Ao. After the two break up, time passes and fate intervenes when Kokoros mother marries Aos father. Suddenly forced to become step-siblings, Kokoro and Ao now must navigate their complicated feelings while living together as a new family. The story explores their transition from former lovers to brother and sister, dealing with the emotional confusion and tension that arises from this unexpected situation.

The setting is contemporary Japan, revolving around the daily life of their newly blended household. Kokoro is the main female protagonist, a young girl attending school who finds herself caught between her past romantic feelings and her new familial role. Ao is the male lead, an older boy who was once Kokoros boyfriend and is now her stepbrother, forced to reconcile their history with their present circumstances.

Published by Shueisha and serialized in the shoujo magazine Ribon, the manga is written and illustrated by Yuko Kasumi. The story fits within the romantic genre, with the central narrative arc focusing on the family merger and the forced proximity of two ex-lovers who must now live as siblings. As of late 2024, the series was ongoing with three volumes released.