She was a tomboy… big emphasis on "WAS".
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](https://www.melonbooks.co.jp/detail/detail.php?product_id=682647))
