恋ようび

Day of Love is about Haruki, the main character, that knew about a girl named Maho when he was young, but forgot about her when he was older, which had made her angry when she asked him years later when he's in high school. So to get back at him for not remembering, she dresses up as a guy and lives in the same dorm as him using the alias Mao.

(from Wikipedia)