Ore To Atashi No Kareshi-Sama

Hayate and Choko are twins who are as different as night and day. Choko acts like a shallow princess whose main concerns are her looks and cute things, while Hayate is the cool but overprotective brother with a penchant for sabotaging potential relationships for Choko. Naturally, Hayate plans to act accordingly when Chouko confesses to the school's prince, Tsukiyono (albeit, mainly for his looks). After an argument over Choko's love life, Hayate and Choko end up tumbling down some stairs together, and bizarrely find they've switched bodies. Even worse, and much to Choko's horror, Tsukiyono starts to fall for Hayate who is now in Choko's body.