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.
He has shoulder length hair with it covering his right eye.
He seems to be dependent on Yume as he tried looking for her many times. This can also be based on the time he spent with no one else but Yume.
He expressed jealousy towards Utsutsu because of Yume's attachment to her brother.
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.