Usagi to Kame Series

Synopsis

During his youth, Itsuki Usawa often took care of Yui Kameyama, Itsuki's neighbor and several years his junior. When she was in elementary school, Yui declared she would become Itsuki's bride. Years pass, and when they meet again, Itsuki turns out to be a teacher at Yui's high school. Will their relationship develop into something more than just neighbors? (Source: MU)