Keroberos, often called Kero-chan or simply Kero by his friends, is the diminutive, orange, plush toy-like creature who serves as the Guardian of the Seal. He was created by the powerful sorcerer Clow Reed to protect a set of magical cards. At the start of the series, he was inadvertently released from a magical book by Sakura Kinomoto and subsequently appointed her as the Cardcaptor, tasking her with recapturing the scattered Clow Cards. He is one of two guardians, with his role being to select and guide the candidate for Master of the Cards.

By nature, Keroberos is bossy, demanding, and incredibly gluttonous, with a particular weakness for sweets like cake, cookies, and chocolates. He is cheerful, talkative, and somewhat of a narcissist, often obsessing over his own perceived coolness, especially when Tomoyo Daidouji films him. In contrast to his more serious and elegant counterpart, Yue, Kero enjoys life’s simple pleasures, including playing video games and eating, though he does not need to consume food to survive as he draws power from the sun. Despite his seemingly carefree and erratic demeanor, he is remarkably intelligent and possesses extensive knowledge of the Clow Cards and various forms of mysticism. He has a strong dislike for sad or lonely situations and prefers to focus on loud, lively fun, using hobbies like gaming and eating as distractions from heavier realities.

In the events of The Sealed Card, Keroberos continues to act as Sakura's primary companion and advisor. A significant part of his role in the film is as a source of comic relief, albeit one that complicates Sakura's personal life. He frequently interrupts Sakura’s attempts to confess her romantic feelings to Syaoran Li, often due to his obsession with food, such as when he smells cake and bursts onto the scene. His presence in the Kinomoto household provides a warm, familial chaos that contrasts with the more serious threat of the film's antagonist, the Nothing Card.

His key relationship remains with Sakura, whom he sees not just as a master but as a close friend. He genuinely cares for her well-being and supports her, even while pestering her for snacks. His relationship with Syaoran Li is one of rivalry and bickering; Syaoran frequently insults him by calling him a stuffed animal or plush toy, to which Keroberos retorts by calling him a brat. However, this mutual antagonism is largely superficial. In the second movie, when the Change Card switches their minds and bodies, they are forced to cooperate and hold an embrace, much to their mutual reluctance but demonstrating an underlying acceptance. Having regained most of his magical powers by this point, Kero does not hesitate to transform into his true, lion-like form to intimidate Syaoran, proving a point about his true strength.

Throughout the film, Keroberos shows no major character development, as his arc had largely concluded in the television series, but he remains a steadfast protector. He provides guidance when Sakura senses the disturbances caused by the Nothing Card, though he is initially unaware of the new threat’s nature. His loyalty and protective instincts are constants; he is always ready to defend Sakura and her friends when danger arises.

In terms of abilities, Keroberos possesses two distinct forms. In his usual borrowed form, he resembles a small, winged, yellow-orange stuffed animal and can fly. In this form, his magical powers are significantly weakened, though he can still breathe small fireballs. His true form is that of a huge, tawny, maneless lion with large, angelic white wings and a metal chest plate and helmet adorned with a large red gem. In this form, his power is fully realized. He has jurisdiction over the elements of fire and earth, allowing him to breathe powerful streams of fire and summon protective energy barriers. His magical power is drawn from the sun and is based on Western magic. He also has a strong ability to sense the presence of Clow Cards and other magical auras, though this can be deceived by sufficiently powerful magic.