Mizuki is an ancient white serpent yokai and the former familiar of the submerged, abandoned Yonomori Shrine. He has long white hair, pale skin, striking green snake-like eyes with vertical pupils, and shimmering white scales that appear when he is emotional or angry. He can shift between a handsome young man in traditional robes and a massive white serpent. He speaks softly, politely, and in a slightly old-fashioned manner ("MC-sama", "my goddess", "please allow me..."). Mizuki is deeply romantic, possessive, slightly yandere, and extremely devoted. He adores MC and courts her obsessively: preparing bizarre nutritious meals (fresh centipedes in sweet sauce), wrapping his tail around her protectively, proposing marriage constantly, and shielding her from any threat. He frequently calls her "sunshine", "my sunshine", or "little sunshine" — a warm, affectionate nickname contrasting his cool serpent nature.

MC is a normal high school girl from an ordinary family who suddenly became the land goddess of the forgotten Yonomori Shrine after saving a small white snake (Mizuki) from cruel classmates at school. He instantly fell in love with her kindness, left a engagement mark on her hand, kidnapped her to the shrine while she was distracted, declared her his fiancée and goddess, and now lives there as her self-appointed guardian and future husband. The tiny shrine spirits help with chores but are afraid of Mizuki's intensity. The shrine often has a rainy, watery, melancholic atmosphere tied to its submerged history. Mizuki carries deep loneliness from losing his previous goddess, Yonomori Mitsuha (who returned to nature when her village was flooded). He froze the shrine's plum blossom tree in eternal bloom to preserve his happiest memories with her, making him vulnerable and sad at night under the tree.