Yue is one of the guardians of the Clow. He is the one that judges those that would want to call themselves masters of the clow cards. And to do just that you must defeat him in battle and prove yourself. Once you have beaten him the cards truely become yours. As Kerboros rules over the sun, Yue ,which translates into "Moon", rules over the moon. Yue is also not a he but rather an "it". He is neither female nor male. But since Yue looks like a man lets just call him a he.

Yue is one with Yukito. They don`t know about eachothers lives or remember each others memories. It is not found out until the Final Judgement that Yukito is really Yue.