The old familiar to a goddess who has long since been dead. He is light-hearted and fun.
When there is a white snake one day in class, Nanami removes it and takes it outside only to have a strange mark on her arm. That mark she later learns is the mark of betrothal to a white snake. That snake then kidnaps her and introduces himself as Mizuki. He turns out to be a pleasent fellow but he's holding her against her will under water where she can't escape. Worst of all the shrine spirits don't understand that humans can't just eat anything raw.
Tomoe then comes to save Nanami and would have killed Mizuki if she hadn't saved him. After he tells them that the reason he'd kidnapped Nanami was because he was lonely and wanted to be a familiar again, she invites him to stay at her shrine.