Giantess in Branwen, the second branch of the Mabinogi. She is the wife of the giant Llassar Llaes Gyfnewid but is twice his size. Matholwch meets the husband and wife carrying the cauldron of regeneration, and because of the disorders of their children, he determines to burn the parents in an iron house. Llassar and Cymidei escape to Britain, where they give the cauldron to Bran in thanks for his hospitality. It is prophesied that Cymidei will give birth to a fully armed warrior. Proinsias Mac Cana thinks that Cymidei has the two attributes of the Celtic goddess: fertility and warlike vigour.