When Clow Reed created his magic cards, he also created powerful beings to help him watch and control them. These were Cerberus, who represents the Sun, and Yue, who represents the Moon. Yue's personality is, interestingly, the polar opposite of that of Cerberus. He is somber, quiet, and even quite grouchy. Although both Cerberus and Yue are very devoted to their master, Yue is almost fanatical in his love for Clow Reed. When Clow Reed tells Yue and Cerberus that he will die one day and someone else will become their master, Yue refuses to accept it. Clow Reed allows Yue and Cerberus to have veto power over the next person who will control the Clow Cards. He decrees that both Cerberus and Yue must approve of their next master. Although this isn't terribly comforting for Yue, he is pacified.
Yue doesn't appear in the story until volume 6 of the manga. Once Sakura catches all the Clow Cards, Yue appears and challenges her. In order to become the new mistress of the Clow Cards, and the only way that Yue will serve her, is if she can beat him in a battle. No one is able to help her in her fight with Yue, not even Cerberus. Yue doesn't hold back in this battle, but luckily Sakura is able to overcome him. He is reluctant, and slightly grouchy, but he does agree to serve Sakura. He protects her in battle and fights along side her, and eventually comes to care for her a great deal. He still has strong loyalty to his former master Clow Reed, however, and things become dangerous when suddenly he can sense Clow Reed's aura again.
Go see Yue in all his bishonen glory!
Spoiler! Go here to find out who Yue really is!