Saijaku Heroine desu ga, Moto Maou na node Miryou de Sekai Seifuku Shimasu

This is a romantic fantasy about a reincarnated former Demon King and Hero. After a deadly battle with the Hero leaves the Demon King in a desperate situation, he uses reincarnation magic in hopes of gathering strength in another world. However, when he awakens, he finds himself in the body of Momozono, a physically weak modern Japanese woman. Panicking because the only magic he can use is a faint charm spell, Momozono soon encounters the Hero, who has been reincarnated as Shirogane, the heir to a major corporation. Seeing that Shirogane does not appear to have awakened his memories of his previous life, Momozono tries to use his charm magic to seduce and manipulate him. Instead, he ends up being doted on and showered with affection by Shirogane.