A snake familiar at the Yonomori shrine. Nanami saved his life from her cruel classmates by releasing him out a window, he fell in love with her, so he left an engagement mark on her wrist. After a day, he arrives at her school and whisks her away to the shrine, which she is unable to escape from. Unlike other snakes, he was apparently raised in a greenhouse and has little knowledge of how most snakes behave. Mizuki is also very lonely because his god has vanished; she once existed when people visited the shrine and believed in her, but now no one lives in the valley and she has dwindled away to nothing. However, he still believes that she exists and will return. As a result, he wants Nanami to live with him forever at his shrine, but when she refuses he tries to rape her and is interrupted by Tomoe. It is revealed that the shrine is an illusion meant to hide his god's favorite plum tree, which Mizuki treasured. Nanami promised to come look at the blossoms with him whenever he felt lonely. Like Kurama he seems to still be harboring a crush on Nanami, and even comes to cook for her when she's ill.
In chapter 23 Mizuki decided to become Nanami familiar making a contract by kissing her while she slept.(Source: Wikipedia)