Keroberos

Cerberus, nicknamed "Kero," is the appointed guardian of the book which holds the Clow Cards. He is one of two magical creatures created by Clow Reed along with the Clow Cards. Before his death, Clow appointed Kero as the one to select the potential candidate to be the next master of the cards, Cerberus himself, and his "brother" and fellow guardian, Yue. After Sakura accidentally releases the cards, Cerberus chooses her to be the candidate and teaches her the basics of capturing the cards. Throughout the series, he displays an extensive knowledge of mysticism. After Sakura passes the final judgment to become the Clow Cards master, Cerberus remains with her as a friend, companion, adviser, and protector when new threats appear.

Having spent a lengthy amount of time in the book while it was in Osaka, Cerberus speaks with a pointed Osakan-accent. He tends to be bossy, demanding, egotistical, and a glutton, but clearly displays his affection for Sakura, especially if she is hurt or in danger. He becomes very fond of video games, and is addicted to sweets. Cerberus spends most of his time in a "false form", a small figure resembling an orange-stuffed animal with wings. Unlike Yue, Cerberus draws his life energy from the sun and is not dependent on the magical power of his master. However, his master must have control of the Firey and Light cards in order to power his true form, a large-winged cat. In the anime adaptation, the Firey card is changed to the Earthy card to avoid to delay his obtaining his true form. Cerberus' name is taken from the Greek mythological figure, Cerberus, a large three-headed beast who was assigned to guard the gates of Hades.

In the anime adaptation, his false form is voiced by Aya Hisakawa, while his hidden form is voiced by Masaya Onosaka. In the English release of the anime adaptation, his name is spelled Kerberos. In the English adaptation, Cardcaptors, it is changed to Keroberos. His false form is voiced by Matt Hill and Richard Newman voices his true form. In the second film, he is voiced by Wendee Lee and Dave Wittenberg, in his respective forms.