Kenshirou Midou is a popular student who harbors a secret: despite being a boy, he has always loved the art of cosmetics. Because of his efforts to keep this low profile, only his sisters and Hiura Mihate, a childhood friend, know his hidden obsession. Apart from his siblings, Hiura is Kenshirou's only hope at realizing his potential.

Kenshirou begs Hiura to let him practice applying makeup on him, and when Hiura finally succumbs to his request, the drastic transformation shocks Kenshirou. From the appearance of a dull, slender boy to a charming petite girl, Hiura's new look evokes something in Kenshirou.

On the other hand, Hiura has had feelings for Kenshirou long before his dramatic change. Often watching from the sidelines, Hiura thinks that a romantic relationship with him is beyond his reach. But with the revelation of Kenshirou's unusual hobby, Hiura remains optimistic and decides to devote himself to his friend's passion. As their affection for one another intensifies, the pair slowly develops an unbreakable bond.

Koisuru (Otome) no Tsukurikata, originally released under the title Osananajimi (♂) wo Onnanoko ni Shiteshimatta Hanashi (幼馴染（♂）を女の子にしてしまった話), is a web manga released on the author's Twitter and pixiv accounts since December 26, 2019. Official serialization started in Comic POOL on February 21, 2020.

The series has been published in English as I Think I Turned My Childhood Friend Into a Girl by Seven Seas Entertainment since June 7, 2022.