Yū

Yuu (ユウ Yū) Is a mysterious man who appears to guide the Hasegawa siblings. Despite appearing human, he's really a monster who has lived on the remote island of Onigashima for generations. He explains to Utsutsu and Yume the reason why they were infected, and that he is searching for his lost mate, who is revealed to be Yume.

Appearance
Yuu appears as a young man, possibly in his late-teens or early twenties, with dark, messy hair and dark eyes. He also wears a dark-colored coat.

He takes on several different forms before arriving at his human one, including a dog and a fly, all while his true body is in stasis. And just like Yume, his true appearance is that of a shape-shifting monster. His larvae form resembles a large caterpillar-like creature that would emerge and be reborn in an eternal cycle.

Personality
As a monster, Yuu has no concept of social norms and may come off as malicious or insensitive. Despite his lack of understanding human emotions, though, he speaks fondly of his partner, deeming Yume as his most important person.

Having consumed each other's flesh countless times for over a millenia, he and his mate had formed a deep attachment. Because of this, Yuu is shown to be quite selfless and self-sacrificing for Yume's sake. When him and his partner are separated, he could only think of her welfare while yearning to meet again. While initially hurt and jealous because of Yume's connection to Utsusu over his own, he later comes to terms with this in order for her to be happy.

History
Originally, Yuu and Yume were two pupa larvae living on an island. However, Ai Imari found them one day 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.