You was born into the famous Shaman family, Asakura. He has a twin brother named Hao who is a powerful reincarnation of the original HAO who keeps getting reborn. For this reason, You is said to be the only one who can defeat his brother, and he himself is, as stated in the manga, a half of the original HAO, thus has many of the qualities of the reincarnation before him. You also has a fiance, named Anna, who he saved from her destructive power of Reishi (mind-reading put simply) which is helped by Hao's old cat friend, Matamune. Matamune, however, dies drastically after saving Anna's life, giving the last bit of his energy to do this. When You was 13, he joined the Shaman Fight, with the reason to make his life a more comfortable place, for both him and Anna.
You himself is a pretty laid-back type of guy. He gets along with everyone around him, and making friends comes easy to him. Although he may not seem serious most of the time, he actually can get really serious when needed to be. Ironically, his twin brother, Hao is a bit like this as well, and many of You's characteristics overlap with his as well. You however, seems to be a bit more sensible than Hao since he looks at things always in a positive light and has something good to say about everything.