Frank Chant was the touchy boy cousin of Christopher Chant, who became Chrestomanci. Though we never learn much about him, it is clear that he married his relative Caroline Chant, also a cousin of Christopher. Frank and Caroline die in a boating accident in the very first few pages of Charmed Life, leaving their children Gwendolen Chant and Eric (Cat) Chant orphaned.
After their deaths, a number of their possessions are given to the children's temporary caretaker, Mrs. Sharp. Among those items are three letters from Chrestomanci to Frank, which suggest that Frank had made some request of Chrestomanci, and later had said something very rude to him, leading to the final letter which broke off their connection. By the end of Charmed Life, Chrestomanci explains the original request to Cat, and tells him why he had never told the boy when he first met him just what Frank's concerns had been. One is left with the impression that the world was a bit better off after the accident removed Frank from the world of the living.