A toy? He sort of looks like one. But he's actually one of the two Guardians of the Clow cards. He rules over the ones under the power of the sun, while Yue-san rules over the cards under the moon. His real form is more like....a lion with wings.

[Sure, Kero could be aglutton and childish at times, but he was always there when she needed him, always helping her and advising her as she caught the various cards over time.]