"Key of the seal.
There is one who wishes the contract with you here,
the girl by the name of Sakura.
Key, give power to the girl.
Release!"
Kero-chan is the guardian of the seal of the Clow Book. He was supposed to be protecting them, but accidently fell... asleep... He's supposed to be the one to choose the new Card Master, to be approved by Yue later. He's a funny little guy though. He LOVES to eat sweets and cakes (and almost about anything sweet. Yukito meets his match!), and always pressures Sakura to bring him some when she goes out (especially if it's to Tomoyo's. Tomoyo makes yummy cakes) He also loves to play video games. It's a good thing, too. I mean, you can't expect him just to sit around all day while Sakura's in school, right?
He doesn't get along at all with Meiling or Syaoran. Kero-chan calls Shaoran-kun "gaki" like Touya does, and Syaoran, in turn, calls him a stuffed animal (therefore recieving a bite or two). Speaking of Touya, Kero-chan has to freeze, pretending to be the stuffed animal Meiling and Syaoran call him, everytime Touya-kun goes into Sakura's room. But that doesn't mean that he doesn't get oniichan's attention. Touya most usually stares at him suspiciously, and Sakura has to divert his attention to something else.
He was created by Clow Reed to protect the Book, but not originally in the stuffed animal form. He always tells everyone that not to underestimate him in his "false form" (well, yeah, it IS a false form), and that his true form is powerful and looks cool. By the end of the second season of the series, Kero-chan regains his power (mainly because all the cards are caught then), and we see his true form. He mostly looks like a lion with angel wings. It still is very cool looking, I guess!
Cerberus cares about Clow a whole lot. I mean, he's his master, right? And now, he also loves Sakura...Of course, she's his mistress now. Sakura, from the very beginning wants Kero-chan as a friend, not just a guardian beast. He helps Sakura every way he can, about and not about Clow Cards. Even though sometimes they get into quarrels, he cares a whole lot about her, and she about him. At the face-off with Yue, Kero-chan was even siding with Sakura, defending her as the worthy Card Mistress.
His favorite color is orange. I think it has to do with his fur when he's in his true form. It is a bit of an orangey...yellowish... color. He also has this adorable little Osaka accent!
Notes about the name:
Cerberus, in Greek and Roman mythology, is a three-headed dog that guards the entrance to Hades...and in the English language, it is an adjective that means "a surly, watchful guard". *sweatdrop* Sure he's a guard, but...
Kero-chan: *glare* But what, web mistress?
Nothing...
*ahem* Sakura was the first one to call Cerberus "Kero-chan". Later, even Tomoyo calls him that. Syaoran and Meiling (and Touya) call him a stuffed animal (much to his distaste). Clow Reed, Yue, Eriol and Sakura (rarely: when she's serious, referring to him as the guardian beast) call him Cerberus ([Keroberusu] in Japanese. The "u" at the end isn't pronounced, and the "o" ever so slightly. So, it's basically Cerberus, too)