Mayo Mayo!

Spin-off of Mayo Chiki! with Hinata Mayoi as the main character.

While using the girl's washroom Mayoi accidentally discovers that the popular and handsome butler Konoe Subaru is in fact a girl. Now that she knows about Subaru's secret, she must work together with Subaru and her sadistic mistress, Suzutsuki Kanade, to protect Subaru's secret from being discovered.

(Source: MU)