Sir Lionel is the younger son of King Bors of Gaunnes (or Gaul) and Evaine and brother of Bors the Younger in Arthurian legend. He is a double cousin of Lancelot and cousin of Lancelot's younger half-brother Ector de Maris (not to be confused with the older Sir Ector, who was King Arthur's foster-father). When their father dies in battle against King Claudas, Lionel and Bors are rescued by the Lady of the Lake and raised in her underwater kingdom alongside her foster-son Lancelot. Like all his family, Lionel becomes a Knight of the Round Table.
While traveling with Lancelot as a young man, Lionel is captured by the rogue knight Turquine, who whips him with briars and throws him in the dungeon. The scenario repeats itself later while he is on the Quest for the Holy Grail, where he proves very unworthy of the blessed object by trying to kill his brother for not rescuing him. Bors had seen Lionel getting beaten and led away, but had to make a decision to save either him or a young girl being dragged in the opposite direction. He saves the girl, and fears Lionel dead. But Lionel escapes, and attacks Bors the next time they meet. Bors proves himself worthy of the Grail when he refuses to fight back, and Lionel kills a religious hermit and Sir Calogrenant, a fellow Knight of the Round Table, when they try to protect Bors from his wrath. Before Lionel can strike his brother, however, God intervenes and immobilizes him.
Lionel and the rest of his family follow Lancelot into exile when the affair with Guinevere is exposed. Lionel participates in the battles against Arthur's forces, and becomes King of Gaunnes. After the Battle of Camlann, Lancelot's family returns to Britain to defeat the remainder of Mordred's forces. Lionel is slain by Mordred's son Melehan; Bors avenges his death.