Kero, along with Yue, is one of the caretakers of the Clow Cards, as appointed by Clow Reed. Despite his intimidating appearance in his true form, he is kind of a slacker at times, loves to overeat, and would rather play video games and watch TV all day than save the world. And he knows it.
Kero, in fact, doesn't need to eat food. He draws his energy from the sun. However, he loves the taste of yummy sweets and eats them regardless of not having anywhere to poop 'em out. ^0^ One of his favorite snacks is pudding. He can be seen throughout the series crying, demanding, moping, and bartering over pudding.
Kero really loves Sakura and their bond throughout the series grows after each episode. He is also very close with Sakura's best friend, Tomoyo. Although he isn't ever all that crazy about wearing her frilly little costumes, it's allowed because of all the great treats she feeds him. Now, as for Syaoran Li, Kero is completely annoyed and aggravated by this kid. It doesn't help Li constantly makes fun of Kero for his puny appearance. He likes Li's fiance, Meiling, even less, and for the same reason. Plus, they are mean to Sakura and are always one step ahead in capturing the Clow Cards!
Kero is a very boisterous personality. He likes to have fun when he isn't working. He is always looking on the bright side and is never sad. He's either happy or sleeping, but for him, there is always a positive side to focus on. This is probably the way Clow Reed created Kero, on purpose. He needed someone that would watch over the cards and never give up, no matter the odds.
As for powers, Kero does in fact have some, despite, again, his puny little appearance. In his "toy" form, he can sense danger, Clow Cards, and can get serious very quickly. He can also fly. Yay! And in very few, rare occasions, he can breathe fire. In his true form, however, he is far more powerful indeed. He can summon magic barriers to guard against attacks and he has the power of the element, fire, on a larger scale. Hence, his firebreath (which is probably what Sakura describes his breath as, the morning after a night of binging!).
Once this occurs, he can transform back and forth at will between his "plush toy" form and his "true" form (a.k.a. BAMF form). When Kero has achieved his true form, Yue is the final guardian that must make his appearance before the sole owner of the Clow Cards is made. While Kero derives his power from the sun, Yue derives it from the moon. This may explain why Kero is such a bubbly personality and Yue is soâ€¦gloomy.
Kero, at this point, is practically family with Sakura. Despite Yue being Yukito, who has always loved and cared for Sakura, Yue forces Sakura to fight him. It's important to note that after Sakura is deemed the sole Keeper of the Clow Cards, Kero remains by her side and even reverts to his false form as "plush toy" Kero. He is second guardian of the Clow Cards, first guardian for Sakura. Which leads me into my final articleâ€¦
I ♥ Kero and I'm so proud to own this shrine to him.