Yuu (ユウ Yū) Is a mysterious man who appears to guide the Hasegawa siblings. Despite appearing human, he claims to be a monster of the who has lived for many generations on the remote island of Onigashima, and explains the reason why Utsutsu and Yume were infected. He is searching for his lost mate, who he claims is Yume.
Yuu appeared as a young man with around Yume and Utsusu's age. He had dark messy hair. Yuu has taken several forms before arriving to his human body, including a dog and a fly, while his true body was in stasis.
Just like Yume, his true appearance was that of a shape-shifting monster. His larvae form resembled a large caterpillar-like creature that would emerge and reborn in an eternal cycle.
As a monster, Yuu had no concept of social norms and may be seen as malicious or insensitive. Despite his lack of understanding of human emotions, he spoke fondly of his partner who has been reincarnated as Yume, deeming her as his most important person. They had formed an attachment to each other after having consumed each other's flesh countless times for over a millennia.
Yuu was selfless and self-sacrificing for Yume's sake. When his partner and him are separated, he only thought of her welfare while yearning to see her. Initially hurt and jealous because of Yume's attachment to Utsusu more than him, he later came into terms with this in order for her to be happy.
He first appeared as an infected dog to Yume and then as a voice by the time they were kidnapped by an organization. He was confused upon knowing Yume does not remember him.