Daughter of Beli Mawr (legendary king of Britain); wife of Llŷr; mother of Bendigeidfran, Branwen, and Manawydan; mother of Efnisien and Nisien by her second husband Eurosswydd. Her parentage links the Llŷr-trio family to the Beli Mawr royal-foundational lineage, making the Branch-2 trio descendants of the legendary king of Britain through their maternal line. The Efnisien-Nisien half-siblings are introduced in the Second Branch as Penarddun's children by Eurosswydd; the half-sibling relationship to the Llŷr-trio drives the central tragedy of Branch 2 — Efnisien's grievance at not being consulted in Branwen's marriage to Matholwch leads to his maiming of Matholwch's horses, the gift of the Cauldron of Rebirth in compensation, the Irish abuse of Branwen, the resulting war, and the catastrophic destruction of both Welsh and Irish forces.