Affectionately known as Kero-chan, he is the Sun Guardian of the Clow Cards and appoints Sakura as the Cardcaptor who will recapture the released Clow Cards. In the English dub of Cardcaptors, he is known as Keroberos.
Created by the powerful sorcerer Clow Reed, Keroberos is one of the two Guardians of the Clow Cards; powerful mystical artifacts that contain potent and potentially destructive spirits of power. Before his death, Clow Reed appointed Keroberos to decide on the next candidate for the bearer of the Clow Cards who would keep them safely contained. In the modern day, he chooses Sakura Kinomoto to serve as the new Card Captor and serves as her guide, companion, and nag.
Keroberos was created by CLAMP and was named after the mythological Cerberus from ancient Greek legend; a large three-headed dog who guarded the gates of the Underworld.
Despite his attempts to find a new wielder of the Clow Cards, Keroberos was unable to find a worthy successor to Clow Reed and ultimately grew bored and fell asleep. Several years later, a 10 year old girl named Sakura Kinomoto discovered the Book of the Clow and opened it, inadvertently releasing the Clow Cards and scattering them across Japan.
Kero was surprised upon awakening from his nap to discover that the cards were missing and immediately appointed Sakura as the new Card Captor, tasking her with the responsibility of retrieving and binding the released spirits of the cards themselves. Keroberos acts as her guide as she finds each mystical spirit and battles them and seals them away and finds herself aided by Syaoran Li, a descendant of Clow Reed and who originally believes her unfit to wield the mystical powers of his ancestor.
Ultimately, Sakura is able to recapture all of the Cards only to find herself tested by Yue, the Second Guardian who is the Judge as to whether or not she is worthy of wielding them and finally accepts her as the true master of the Clow Cards.
As the Guardian Beast of the Clow Cards, Kereberos possesses the power to sense the presence of the Clow Cards; he can tell if they are nearby and if they are active or in use. He can also sense magic as well. Only the most powerful sorcerers can block or interfere with his senses. Unlike the other Guardian, Kero is able to sustain himself due to the fact that he draws his power from the Sun without drawing upon Sakura's magic. Thus, he does not actually need to eat or drink; he simply does so because he is a glutton and enjoys the finer things in life.
In his regular disguised form, Kereberos appears to be a small anthromorphic lion doll with a pair of wings which is regularly referred to as his "plushie" body. Previously, Kero had little or no powers in this state save for flying and his sensing abilities. The more Clow Cards that Sakura has in her possession, the more powerful he becomes. In his plushie form, he was later able to breathe tiny bursts of fire.
When Sakura has possession of the Firey and Earthy Clow Cards are his full powers unlocked and he can assume his true form; that of a large leonine body with jeweled armor and a pair of magnificent feathered wings. In this form, he possesses the power to command both fire and earth as they fall under his jurisdiction. He can create powerful protective shields and shoot intensely powerful fireballs from his mouth.
Keroberos also possesses a tremendous amount of mystical knowledge and has become a rather talented player (and addict) of video games.