Lionel's family belongs to the neighbouring royal houses of Ganis and Benoic. His father, King Bors the Elder, is Ban's brother, and his mother Evaine is the sister of Ban's wife Elaine. Lionel and the younger Bors are therefore related to Lancelot through both parents. In the French prose childhood narrative, the fall of their father's kingdom places them in danger from Claudas. A loyal knight, Pharien, protects them; their conflict with Claudas's son Dorin leads to another crisis, and an agent of the Lady of the Lake brings the brothers into her protection. Their upbringing connects family loss with the enchanted refuge already associated with Lancelot.

At Arthur's court Lionel becomes a knight of the Round Table. In Malory, he accompanies Lancelot on a journey and watches while his cousin sleeps. Seeing a powerful knight capture three opponents, Lionel pursues and challenges him. The attacker, Turquine, defeats him as well. The captives are stripped, beaten with thorns and placed in a dungeon containing other knights. Lancelot later overcomes Turquine and ends the imprisonment. The episode establishes Lionel as a brave participant in adventure who can nevertheless require rescue from a stronger opponent.

A different captivity appears during the Quest for the Holy Grail. Bors sees Lionel being driven along naked and beaten by two knights, but also sees a maiden being carried away under threat of sexual assault. He chooses to save her and entrusts his brother to God's protection. Lionel survives, yet interprets the decision as a betrayal. When the brothers meet again, Bors greets him joyfully and then pleads for mercy when he understands his anger. Lionel refuses the appeal and attacks him.

The quarrel grows beyond the brothers. Lionel kills the hermit who tries to shield Bors and then kills the intervening Round Table knight called Colgrevance in Malory, a form associated with Calogrenant. Bors initially refuses to fight, but eventually draws his sword to defend himself. A warning voice stops him, and a fiery cloud comes between them, burning both shields and leaving both men unconscious. On recovering, Bors asks forgiveness, and Lionel grants it. Malory therefore gives a more complicated scene than a permanently passive Bors and a single brother immobilised by fire.

Lionel later follows Lancelot when the conflict over Guinevere divides the court. Loyalty to his cousin draws him into war against Arthur and into the distribution of lands overseas. French prose tradition preserves an ending in which Melehan, a son of Mordred, kills Lionel and Bors takes revenge. Malory instead states that Lionel takes fifteen lords to London while searching for Lancelot and is killed there with many of them. The killer is not named in that notice. These are alternative endings, not successive adventures. Lionel's story leaves a family diminished by both external enemies and violence within its own bonds.