Created by a powerful sorceror named Clow Reed, Cerberus was given the power of the sun and became one of the two guardians of the Clow Cards, the other being Yue of the moon. It was Keros job to find the person who would inherit the Clow Cards and keep their magical powers in check. He eventually found Sakura and gave her a magical wand to set her off on her quest. Because the Clow Cards had been set free, Keroberos had no power left and was forced to assume the identity of what looked like a stuffed animal. Luckily for him, his power grew each time Sakura got a card and, after catching the Firey card, Kero could assume his true form, a large lion-like create with massive angel wings. It also allowed him to control six of the clow cards and breath fire. Once Sakura caught Earthy, Kero had all of his magical powers back.
Because Kero had the power of the Sun, when his magic was restored he didn't need to rely on anyone else for magic like Yue because he had his own unique magic circle.
Flight - Kero is a skilled flyer throughout the series and the comic, he is easily one of the most skilled in the story.
Fire Blasts - Because Kero's offensive element is fire, his main attack is a powerful fire blast that was only matched by Spinel Sun's energy blasts. It is unkown who else could block this attack other than Clow Reed and possibly Yue.
Shields - Like all guardians, Kero has been shown to create powerful shields that are almost impermeable to things other than high class magic such as Clow Reed.