Plot
He appears in Chapter 13, where Ashley's followers surround him. As Refi gets scolded by Ashley with Sion, Yuu recognises Refi, much to his surprise. Elulu then says that he's the child prodigy that the principal asked.
Stats and Abilities
Yuu possesses the highest magic throughout the whole game, using dark offensive magic. He mainly uses AOE, and doesn't have any stat spells.
Ending
In Yuu's ending, he's a detective who's trying to find the culprit to a crime. Refi is his subordinate, who tells Yuu to find, that he needs to go to a nunnery. Yuu has the crossdress to enter, whil Refi laughs at him, saying how it fits him perfectly, and he's the only one who can do it.