Zutto Otokonoko da to Omotte ita Gakitaishou ga Onnanoko deshita

Also known as: ずっと男の子だと思っていたガキ大将が女の子でした, The Gang Leader Who I Thought Was A Boy Is Actually A Girl

by Chamirai

When he meets his childhood friend again, the one he thought was a boy, he discovers that she was actually a girl, who has grown up to become a beautiful young woman! She takes care of him, they go on a date to the park together, and there are lots of sweet and sour situations! (Source: Melonbooks, translated)