Yue is Yukito's true form. He is the judge of the Clow Cards, and thus is responsible for making the determination of whether or not a person is worthy of owning the cards. He's the judge that will approve Sakura's right on the Clow Cards when she captures them all. As Sakura collects the last of the cards, he challenges her to a battle. Sakura manages to prove herself and Yue agrees to serve her so he's also Sakura's guardian, together with Kerberos. His symbol is the Moon, as opposed to Kerbero's, the Sun
Yukito does not remember any portion of his life as Yue, however Yue does retain knowledge of his life at Yukito
*The name'Yue' is the Chinese word for 'moon'*