Meriadoc of Cambria
The King of Cambria (Wales) in the anonymous romance bearing his name.
Meriadoc had a sister named Orwen. Their father, King Caradoc of Wales, was murdered by Caradoc’s brother Griffin who assumed the throne for himself.
Griffin plotted to kill the children but they were taken to safety by their foster-father Ivor the Huntsman and his wife Morwen. Urien (here called the King of Scots), abducted Orwen and married her. From Ivor, Meriadoc was kidnapped by Kay, who took him to Arthur’s court. Under Kay’s tutelage, Meriadoc grew into a powerful warrior, and he championed Arthur against the Black, Red, and White Knights.
Arthur rewarded him by giving justice to Griffin, which allowed Meriadoc to claim the throne of Wales. He turned Wales over to the stewardship of King Urien, his brother-in-law, and left Britain for Europe. There, he joined the forces of the Emperor of the Alemanni against King Gundebald of the Land From Which No One Returns, who had kidnapped the Emperor’s daughter.
He braved a number of supernatural adventures and finally slew Gundebald. He rescued the Emperor’s daughter, fell in love with her, and returned with her to the Emperor. The Emperor, however, wanted to betroth his daughter to the King of Gaul, so he betrayed and imprisoned Meriadoc. With his lover’s assistance, Meriadoc escaped and slew the Emperor.
Meriadoc was awarded lands by the King of Gaul and ruled them nobly for the remainder of his life, with the Emperor’s daughter as his queen.
Source
Historia Meriadoci Regis Cambrie | Late 13th century