Shoujo to Toshi no Sa, Futamawari.  

Masami is an officer worker who has lived a relatively uneventful life. His parents passed away, he was hospitalized for a while, then he changed jobs due to these events. Despite various things happening, he mostly lived in the moment. It wasn't until he realized that others were starting to call him "oji-san" at his age that he began to feel lonely. He didn't have any friends or a girlfriend and kept a distance from his colleagues at work. However, he did have a cousin Rin who often called him and they enjoyed talking. Rin worked as a hostess at night and left her teenage daughter Mizuka alone at home every night. She asked Masami to take care of Mizuka whenever he had time. As a result of this trivial offer, their loneliness gradually decreased.