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.

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 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 delaying 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.