Kero-chan's personality is pretty complex in its... kero-chan-ness. He has been alive for many, many years (even though he slept through 30 of them ^^;) so he is very wise and can handle himself well in difficult situations. He is also very knowledgeable and can successfully guide Sakura through the trials of being a card captor, sometimes even serving as sort of parental figure to her.
But what truly makes him the Kero-chan we all know and love are probably his weaknesses and little quirks.
Number one among them is his extreme fondness for sweets and other foods (especially pudding). Even though he doesn't need to eat and in spite of his small size, Kero-chan tends to consume large amounts of sweets with extreme joy ("pudding ya, pudding!!"). He usually depends on Sakura to bring him dessert, so he's devastated whenever she can't save him any.
Besides food, Kero-chan is extremely passionate about his favorite pastime: playing video games. The fact that Sakura's controllers are larger than himself doesn't seem to deter him, and it's probably safe to say that to Kero-chan there are probably few things as crushing as losing a saved game.
Although Kero-chan is very small while in his false form, his presence naturally draws attention. All of his emotions tend to be very strong whether he's happy, excited, angry, or sad, so he tends to be pretty loud in spite of his size. But that's probably okay with him, since Kero-chan loves attention. He's also very confident, so even while in his cute smaller form, he always thinks of himself as the impressive Beast of The Seal (and during the time he can't regain his true form, he keeps himself happy by talking about how cool his Kerberos form is).
Knowing this, it's not surprising that nothing hurts Kero-chan more than being called a stuffed animal ("nuigurumi," to quote Syaoran -- let's just say he's not Kero-chan's favorite person). Kero-chan usually reacts to personal attacks pretty strongly, especially if you hurt his pride. Although I guess that to say that he "reacts strongly" is a sort of understatement... just ask Syaoran, who's had his fingers bitten more than once by a certain set of angry little teeth. Nevertheless, Kero-chan is very friendly if you don't make fun of his size or doubt Sakura's abilities as a card captor.
All his pride could make Kero-chan seem arrogant, but since he's about 5 inches tall most of the time, his high opinion of himself is quite adorable and amusing. Besides, from his confidence (and profound love of sweets) comes the energetic, loud, and happy little bundle of joy that is Kero-chan. ^_^
