Keroberos, almost always called Kero-chan or simply Kero by those close to him, is one of the two magical guardians created by the sorcerer Clow Reed to protect the Clow Cards. His formal role is the Beast of the Seal, and he is also responsible for selecting a new master to inherit the cards. For many years, he slept within the Clow Book, which was kept in Osaka, and this long stay is the reason he speaks in the Kansai dialect. He remained asleep for around thirty years until the book was accidentally opened by Sakura Kinomoto, who became the new Cardcaptor.

In his everyday appearance, Kero-chan resembles a small, plush, orange-yellow stuffed animal with small white wings, large ears, and a lion-like tail. This form is often mistaken for a doll or a plush toy. His true form, however, is a majestic, maneless lion-like beast with enormous white feathered wings and a metal chest plate adorned with a red gem. In this true form, he is a powerful and imposing creature.

Kero-chan has a very outgoing and cheerful personality. He is energetic, talkative, and can be quite boastful and narcissistic, often obsessing over his own perceived coolness. Despite his small size, he is demanding and has a massive appetite, with a particular love for sweets, cakes, and snacks like takoyaki. He also loves playing video games and enjoys being filmed by Tomoyo Daidouji, happily wearing costumes for her recordings. His favorite things are loud, fun activities and bright colors like red and orange, while he deeply hates anything sad or lonely. Although he appears carefree and gluttonous, he is remarkably intelligent and deeply knowledgeable about magic, the Clow Cards, and mysticism.

His primary motivation is to guide Sakura in her duty as the Cardcaptor, ensuring she successfully recaptures all the Clow Cards and passes the Final Judgment to become the new master. He takes this responsibility seriously, even if his demeanor is often lighthearted. While he has a genuine care for Sakura and believes in her potential, he also fears the consequences of failure, particularly the judgment of his fellow guardian Yue. His role in the story is to act as Sakura's mentor, confidant, and protector. He provides her with crucial information about each card, helps her develop strategies, and is her first and most constant ally in her magical adventures. He also serves as a source of comic relief and daily companionship, living in a small bed inside Sakura's desk drawer.

Kero-chan's most important relationship is with Sakura Kinomoto. While he is technically her servant as the master of the cards, their bond is much closer to a deep friendship, and she affectionately calls him Kero-chan. They occasionally argue, often over his demands for food, but they care for each other immensely. He also has a close relationship with Tomoyo, who often bribes him with sweets and films him. His relationship with Syaoran Li is initially antagonistic, as Syaoran calls him a stuffed animal or plush toy and Kero calls him a brat; they maintain a competitive rivalry throughout the series. His most complex relationship is with Yue, the other guardian of the Clow Cards. They are partners and have known each other the longest, but Kero can be somewhat jealous, feeling that both Clow Reed and Sakura favor Yue. Despite this, they trust each other and Kero shows deep concern for Yue's wellbeing.

Kero-chan experiences notable development throughout the story. Initially, he is trapped in his small plush form and cannot access his full power. He is able to regain his true form only after Sakura captures the Earthy card, which, along with the Firey card, is under his jurisdiction and symbolizes his power over the sun and the earth. As the series progresses, he learns to trust Sakura's growing abilities completely, and his role shifts from a strict guardian administering a test to a loyal and supportive friend. When Sakura becomes the new master, he remains with her not out of duty, but out of friendship.

As a magical being, Kero-chan has several notable abilities. His power is drawn from the sun, which makes him largely self-sustaining, unlike Yue who requires magical energy from their master. In his small form, he is capable of flight and has a weakened ability to sense magical presences and the Clow Cards. In his true form, his powers are greatly enhanced. He can breathe fire, project powerful energy barriers, and use various abilities related to the Clow Cards under his jurisdiction, which include the Firey and the Earthy. He is also a key figure for the Clow Cards, as half of the deck, specifically those related to the sun and eastern magic, draw power from him. Furthermore, he can replenish his magical energy simply by eating food, which explains his constant desire for snacks.