My Friend's Little Sister Has It In for Me! Volume 10

The first time Akiteru met his friend’s little sister, Iroha, she was soft-spoken, with a diligent air about her. So when he heard she was interested in joining a gang—one led by none other than Otoi—he began to suspect there was more to Iroha than met the eye. When he intervened, things only got more complicated. And in the end, Akiteru formed bonds with unlikely friends for years to come. It’s finally time to unravel the secrets of the Alliance’s past! How did Akiteru come to form the 05th Floor Alliance, and why did Iroha decide to make him the victim of her teasing?

[From J-Novel Club]