Eriol Hiiragizawa/ Eli Moon is from Great Britain. He transfers to Japan and becomes a classmate of Sakura. He is very courteous, but he bothers Li Shaolan, whom he calls his "cute/small relative!" He resembles both Fujitaka Kinomoto/ Aden Avalon, Yukito/Julian and Clow Reed.
Eriol/Eli has magical powers and a staff with a sun on the end, which looks an awful lot like Sakura's Key. He lives with and is aided by, his guardians, Ruby Moon and Spinel/Spinner Sun.
Eriol/Eli was not the normal boy Sakura Kinomoto took him for. Infact he was far from it. Eriol/Eli, 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 to face. This makes him seem like the enemy, but the fact remains that everytime Sakura uses a card now, it changes and becomes stronger. Eriol/Eli is merely helping Sakura.
He is, as I have already stated, the reincarnation of Clow Reed, the incredibly powerful magician who himself created the Clow Cards. Clow Reed, upon his death, decided to split his soul, including his powerful magic, into two human beings. This was a great idea because, just think about it, how awful would that be to have all that power to yourself? You couldn't really do anything with it, except maybe destroy the world or something of a catastrophic nature, because no one could parallel you. Eriol/Anthony and Fujitaka/Aden are the reincarnations of Clow Reed. Fujitaka/Aden himself has Eriol/Anthony's shadow.
Now, the magic of Clow Reed comes from both Chinese and English backgrounds, making it doubly mystical in the sense that both of these ancient cultures believed in extreme powers, such as those described in the Clow Cards. So just in case you had been wondering why Eriol/Anthony was an Asian-looking boy born in England, now you know. Well, Eriol/Anthony created two guardians for himself while searching for his way around the powers he inherited from his past life. They would be Spinel Sun and Ruby Moon. These two are like Kero-chan and Yue, the guardians of Sakura, one from the sun, another of the moon. Anyhow, Eriol/Anthony realises that the only way that he can mend his past mistakes and evenly split his powers would be to find the one who could work the magic of the Clow better than he could.
Thus, he learns of a girl named Sakura. In order for him to test her powers, he challenges her to a "battle." But her powers have not fully matured yet, so he aids her. Eriol/Anthony teaches Sakura to make the Sakura Clow, and her powers grow.
Many people think that Eriol/Anthony sort of has a thing for Sakura around this time in the plotline, however he is genuinely trying to help her and maybe trying to make Li jealous. He needs this girl's help to make things right, so naturally, he watches and plots her progress.
He also knows that there is another reason that Sakura has these powers. Fujitaka/Aden, her father. Fujitaka/Aden is, or was, to have the other half of Clow Reed's magic. So this inspires Eriol/Eli further to make sure that Sakura and her powers are the best.
His is complex, but I will try my best to explain it as best I can. Although he is a young boy, he has mannerisms and ways about him that most definitely reflect those of a grown man. Clow Reed shining through. Eriol/Eli is quite the charmer, to say the very least. Just about everyone he meets is immediately taken by his charisma and looks. He is the type that will always be polite, addressing everyone with their respected title, and speaking with flawless grammar. But he is prone to lying, anything to get a good response from whomever he is conversing with. But despite the front he puts out, there is a much deeper and sadder side to Eriol/Eli which many, by simply chatting with him, would find hard to believe. Yet I tend to believe that his expressions give him away. He always looks so melancholy, even when he's smiling.
He absolutely hates the fact that he has all of his past powers. Therefore, he can always stick to business when it comes to his mission. He is miserable, and just wants it all to be set straight, for Fujitaka/Aden to receive his share of the magic, and for his life to resume. What a burden this little boy has to carry on his shoulders. You sort of can't help but pity him a bit.
When Eriol/Eli first appears, it seems that he is the villian. He looks the part, and he certainly is shrouded in mystery, but after awhile it's obvious that his intentions are for the better. It must have been really hard for someone with such enormous power at his command to keep it all sealed away and not be tempted into using it for evil.
All in all, Eriol/Eli is a misunderstood, yet quite charming little boy who tends to be a bit over-flirtatious. This is the reason why Li Shaoran develops hatred towards him when he first appears (he kisses Sakura's hand, and Syaoran has a fit!).