Yui is the daughter of human and a Shi (another type of demon, also described as anti-shinma in some English translations). She is also part vampire as Miyu's blood runs in her veins, as Miyu fed on her mother while pregnant with Yui. Miyu describes Yui as both her sister, her daughter, and herself. Yui has long dark hair, and looks to be about twelve or thirteen years old. Like Miyu, Yui has a tragic past and friends she had to abandon upon her awakening. Yui has the additional issue of being both hunted and supported by rival factions of Shi, who do not approve of her existence.

Also like Miyu, Yui also has a blood bonded protector, a younger shinma named Nagi. Yui's personality is much more shy and timid than Miyu's, and she is still largely naive about the world at large. Nagi is very hot headed, but is a faithful companion, allowing Yui to feed on him as she refuses to feed on humans. Yui's weapon of choice are cherry blossoms, which she can bend to her will in a variety of ways. She passes this power to Miyu upon giving up her own blood for Miyu's rebirth.