Pinel le Savage
For "pure envy and hate", Pinel tried to poison Gawaine and avenge the death of Lamorak. The poison accidentally killed Sir Patrise of Ireland instead. After lying low while Mador de la Porte impeached Queen Guenevere for Patrise's death and Lancelot fought to save her life, Pinel "fled into his country" when Nimue uncovered the truth.
In Vulgate VI, the name of this mortal enemy of Gawaine's is Sir Avarlon. As Pinel in Malory, so Avarlon in the Vulgate tries to poison Gawaine with apples at a small dinner party given by the Queen. Here, Sir Gaheris de Kareheu falls accidental victim (do not confuse this Gaheris with Gawaine's brother).
The Poisoned Apple | The Legend of King Arthur