Kid Mon: Real Name Unknown

少年モン、本当の名前は知らない

Also known as 没有名字的少年

In a small town with a big red bridge, a run-down apartment, which everyone calls a haunted house, stands still. A few days before the summer break begins, two high school girls, Kaede and Azusa, find a boy living in the abandoned place by himself. Naming him "Mon", they take care of him without being noticed by anyone.