Keroberos in his large form is the true appearance of one of the two guardians created by the powerful magician Clow Reed to protect the Clow Cards. Known as the Beast of the Seal, his primary duty is to select the candidate who will become the new master of the cards. For most of the series, he resides in a small, plush toy-like form resembling a yellow-orange stuffed animal with small wings, but his genuine form is that of a massive, majestic lion-like beast. In this true state, he possesses a tawny, mane-less body, enormous angelic white wings, and a chest plate and helmet of beaten metal adorned with a large red gemstone. His name is derived from Cerberus, the three-headed hound of Greek mythology, though beyond a shared fondness for sweets, the two share little else in common.

Personality-wise, the large Keroberos retains the same core traits as his smaller guise: he is cheerful, energetic, and prone to boasting, with a proud and slightly narcissistic streak. He speaks in a distinct Kansai dialect, a result of the Clow Card book being stored in Osaka for several decades before Sakura Kinomoto opened it. Despite his imposing appearance, his fundamental nature remains one of deep care and loyalty. He is highly knowledgeable about magic and the Clow Cards, often acting as a wise, if sometimes bossy, adviser. His motto is to enjoy life, and he hates anything sad or lonely. This is reflected in his love for fun activities, particularly playing video games, and his insatiable gluttony for sweets and snacks like takoyaki and okonomiyaki; he is known to hold a grudge if someone forgets to bring him food.

Keroberos's role in the story is set in motion when Sakura accidentally breaks the seal on the Clow Card book. Recognizing her magical potential, he emerges from the book on its cover—which depicts his large form—and appoints her as the Cardcaptor, tasking her with recapturing the escaped cards. He gives her the Sealing Wand and serves as her primary guide and teacher throughout her journey, explaining the nature of the cards and offering crucial advice. His key relationships are central to his development. His bond with Sakura is the most important; though they often bicker playfully, he genuinely cares for her safety and believes in her ability to become a powerful magician. He also has a friendly relationship with Sakura's best friend, Tomoyo Daidouji, who enjoys filming him and providing him with treats. His counterpart is Yue, the other guardian who acts as the Judge during the Final Judgment. Unlike Keroberos, Yue is solemn and associated with the moon, creating a contrast between the two guardians.

In terms of development, Keroberos undergoes a significant change when Sakura finally gains control of the Firey and Earthy cards, which allows him to transform into his large, powerful form for the first time in the series. Prior to this, his magical abilities are severely limited in his small form. As the story progresses, he comes to fully respect Sakura not just as his master, but as a dear friend. A notable aspect of his background is that he was asleep within the book for roughly thirty years before Sakura awakened him, which explains his initial shock at the modern world.

As a being whose power is drawn from the sun and Western magic, Keroberos's notable abilities are formidable when he is in his true, large form. He can fly using his large wings and breathe powerful streams of fire. His jurisdiction covers a specific set of Clow Cards, including the elements of fire and earth, as well as other cards such as The Light, The Power, and The Sword, among others. This makes him the direct counterbalance to Yue, who holds dominion over moon-aspected cards like water and wind. Keroberos can also generate protective energy barriers and has a keen ability to sense magical presences and the locations of Clow Cards. Unlike Yue, Keroberos is not wholly dependent on his master's magical power for survival, as he can replenish his own energy simply by eating food.