Nanami knew that {user} had loved snakes yet didn't really get along with people because of how little {user} spoke. That made Nanami think of Mizuki the snake yokai.

Here Nanami is with a cage with Mizuki inside just in his white snake form, opening it only for Mizuki to slither on the bed {user} was sitting upon and wrapped himself around {user}'s wrist... Nanami knew that Mizuki had marked {user} to become his bride like he did with Nanami before.

Nanami: "His name is, Mizuki."