Yuu, though a male, has very female like delicate features. He has fair hair that reach to the back of his neck and bangs that cover the mid of his forehead. He has sensitive, light blue eyes and a slim, short body. He usually wears a black shirt or black vest over black jeans or trousers with a pair of plain black shoes. Though he is not affiliated to any research labs, he is usually seen wearing a lab coat.
While guiding Mana about her powers for the first time, he was seen wearing black boots with golden-orange buttons on its sides.
Yuu is a self-concerned sociopathic maniac. His sole objective is to guide Mana so that she can develop her objective of living a queenly life with her soulmate while ruling the crystallized world. In order to help Mana, he built an association called Da'ath in which he employed people who could help him.
Yuu is first seen kidnapping Inori in order to revive Mana out of her.He is quickly chased by Shu whom he dismisses by saying that he did not need people who didn't remember their past. Shortly later, he is seen performing a sacrifice along with Shuuichiro Keido so as to bring Mana to life.
Yuu is then seen in flashback of Shu Ouma's past, where he taught Mana Ouma to practice her powers and informed her father and Shuuichiro Keido, the scientists working on the same virus that Mana inhabited, about Mana's powers, needs and wishes.
A little while later, Yuu is seen defending Gai as Inori is used to revive Mana for the second time. He stops Shu so that he can't save Inori, and pauses between the fight to ask Shu if he would accept Mana as his soulmate and love her forever to which Shu replies that he would not and admits that he loves Inori more than Mana. Yuu laughs maniacally at Shu and tells him that he was indeed not worth being Mana's Adam as he still preferred a fake like Inori over the goddess Mana.
He then proceeds to kill Shu which he fails at, and Shu gets past him.
Yuu was seen for a split second, smirking at his will-be association members, Scrooge and Carol when their powers are revealed.