Catherine was the second daughter of Constance, Duchess of Brittany, and Guy of Thouars.
Catherine had a twin sister, Margaret, and their mother might have died because of a difficult delivery[1][2][3][4][5][6][7][8] After her mother's death, her father married Eustachie of Chemillé, and had two sons, Peter and Thomas.
Catherine was the younger half-sister of Eleanor, Matilda and Arthur, Constance and Geoffrey of England's children, the sister of Alix and Margaret, and the elder half-sister of Peter and Thomas of Chemillé, Guy and Eustachie of Chemillé's sons.
They had three children: Catherine of Thouars appears as a secondary character in the novels Le Poids d’une couronne (légende bretonne) (1867-1868) by Gabrielle d’Étampes and El Viaje de los Siete Demonios (1974) by Manuel Mujica Lainez.
She is also mentioned in the novel Dans l’Ombre du Passé (2020) by Léa Chaillou, where it is revealed that the heroine's sister is named after her.