Like Hiei and Kurama, Yusuke is from the anime/manga Yu Yu Hakusho. In short, he's one of the main guys of the show and leader of the small group. He's also half demon(?) or so.
In Elyasia, Yusuke, through his demon blood, is linked to the princesses. They called him Cousin but the true link is unknown. Anywho, Yusuke will appear in the story for short times to help things go. And to add funny parts.
Yusuke is mated/married to Kita and has one child, Kit.