Key Mystic Undercover

Haruto, a university student, works part-time as the exclusive model for Keymistic, a women’s lingerie brand run by his aunt. His face and name are kept hidden, and only his back is ever shown in public, making him a mysterious figure who’s gaining attention.
But the fact that he’s a man is an absolute secret that no one is supposed to know.

One day, after a photoshoot, Haruto is caught changing clothes by popular model Mashiro, who discovers his true identity.
In exchange for keeping quiet, Mashiro makes an outrageous demand to let him touch Haruto's butt...?

(Source: Takeshobo, translated)

Note: Includes the previously self-published manga "Shite yo Sasete yo (してよさせてよ)" and four extra chapters.