Royarch

The Royarch is the King of Bretonnia, a Grail Knight of the Lady of the Lake, and a holy-warrior who is the very pinnacle of Justice and Chivalry. The Royarch is chosen from either a successor to the original Royal Blood-line or chosen by one of the twelve Dukes of the twelve Duchies. The first Royarch was the legendary warrior and uniter Gilles le Breton, who was crowned by the Fay Enchantress as the first Royarch and founder of the Kingdom of Bretonnia. The current Royarch, Louen Leoncouer was crowned by the year of 1522 (2500 IC).

Source

 * Warhammer Armies: Bretonnia (5th Edition) -- p. 48, 78 
 * Warhammer: Knights of the Grail (FRP) -- p. 30 - 31