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, Kero remains with her as a friend, companion, adviser, and protector when new threats appear.
Source from Wikipeida