Kyuujitsu Gentei Kanojo

Haruko Shiina is an elite office lady who enjoys being around "cute girls" every day, which she achieves by mentoring junior colleagues at work… until a handsome and popular colleague, Yuzuru Matsui, takes that role away from her. One day, on her day off, Haruko is hanging out at a crowded café when she finds Matsui in the middle of a heated argument. He turns out to be an unscrupulous playboy in private, which only makes her even more wary … "Shiina-san, please go out with me." "No, I can't." Haruko immediately rejects Matsui's sudden confession, but he refuses to give up so easily. In order to fend off the narcissistic and prideful Matsui, Haruko sets an impossible condition, agreeing to go out with him only if he "dresses up as a woman!" A romantic comedy about a love triangle(?) formed between two people! (Source: Pixiv Sylph, translated)