Eriol Spoilers
Eriol was not the normal boy Sakura Kinomoto took him for. Infact he was far from it. Eriol, as confirmed by Yue, is and was the legendary Clow Reed, creator of the Clow cards.
He is also the one who is creating new enemies for Sakura-chan to face. This makes him seem like the enemy, but the fact remains that evertime Sakura uses a card now, it changes and becomes stronger. Perhaps Eriol is merely helping Sakura.
It was then revealed that Eriol in not wholly Clow!! It seems when Clow died he was reborn into to individuals: Eriol and Fujitaka Kinomoto!! Fujitaka himself has Eriol's shadow.
Eriol is aided by his guardian's Ruby Moon and Spinel Sun, the former posing as a female student in Yukito and Touya's class (and she won't leave the two in peace ^_-).
