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 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.
Originally Yuu and Yume were two Pupa larvae who had been together living in an island. However, one day Ai Imari found them while taking a stroll and took them to her laboratory in order to inflict experiments on them. Yume managed to escape and impregnated Sachiko Hasegawa, therefore reincarnating as a human girl. Alone, Yuu kept on being experimented.
He first appeared as an infected dog to Yume and then as a voice by the time they were kidnapped by an organization. After saving Utsutsu and Yume from the organization, Yuu invites them to live with him while they're on the run. He also tells Utsutsu that Yume is his lost mate who was separated from him years ago. However, Yume doesn't remember him, so Yuu decides to make her remember with his powers. Unfortunately, this makes Yume transform into a monster and cause a rampage, until she's pacified by Utsutsu. This makes Yuu realize that Yume loves Utsutsu now and comes to terms with it.
The "Dearest You" extra-story reveals that Yume did regain her memories after Yuu's attempt. She apologizes for leaving him behind at the lab all those years. Yuu accepts her apology and warns her than an eclipse will come soon and they need to "emerge" and leave their Pupa forms. However, Yume refuses, since she doesn't want to leave Utsutsu behind. Yuu says goodbye to Yume, since apparently something had happened to his body.
It's revealed that at some point during their stay at the island, Maria had killed Yuu and his body had been assimilated by the Pupa Larvae. However, he's still able to communicate with Utsutsu mentally. Yuu tells him that Yume needs to emerge before the eclipse comes, but she needs a partner to do so. Utsutsu accepts this and he and Yume emerge at the eclipse, as their bodies dissintegrate into butterflies.