Sir Lionel was the son of King Bors of Gaunes the cousin of Lancelot in the Vulgate Cycle, but in Morte Darthur was the nephew of Lancelot. He was named Lionel at birth, because his mother Evainne saw a strange birth mark on his chest, in the shape of lion. Lionel became Lancelot's squire during King Arthur's war against Galehaut. Lionel was knighted after Arthur's war against the Saxons at La Roche (the "Rock").
Lionel, like the other knights of the Round Table, took part in the Holy Grail quest. Lionel was captured by two knights where he was badly beaten. His brother Bors had to choose either to save him or a damsel who was about to be raped. Because his honour as a knight demanded that he save a woman before a fellow-knight, even if the knight in trouble was his own brother. Bors rescued the damsel. Bors discovered later that his brother had managed to escape from his tormentors. Lionel selfishly accused his brother of saving the damsel but not him. Lionel challenged Bors into a duel, who refused to his own brother. A hermit, who tried to intervene and save Bors, was killed. Calogrenant, a fellow-knight had also tried to intervene on behalf of Bors, but he was no match for Lionel, and he too died in combat. Only God saved Bors from Lionel's vengeance by hurling a lightning bolt between the two brothers. Not till then did Lionel realise his error. Lionel buried Calogrenant and the hermit, while Bors continued on his quest, meeting Perceval and Galahad.
Lionel sided with his cousin Lancelot, when there was conflict between his kinsman and Arthur. Lionel took part in the rescue of Guinevere, the war at Joyeuse Guard and the war in Gaune, France.