Lancelot du Lac
One of the earliest romances of the “Round Table” (1494). Sir Lancelot was the son of King Ban of Benwicke, but was stolen in infancy by Vivienne, called “La Dame du Lac,” who dwelt “en la marche de la petite Bretaigne;” she plunged with the babe into the lake, and when her protégé was grown into man's estate, presented him to King Arthur. The lake referred to was a sort of enchanted delusion to conceal her demesnes. Hence the cognomen of du Lac given to the knight. Sir Lancelot goes in search of the Grail or holy cup brought to Britain by Joseph of Arimathe'a, and twice caught sight of it. (See Graal.) Though always represented in the Arthurian romances as the model of chivalry, Sir Lancelot was the adulterous lover of Guinevere, wife of King Arthur, his friend. At the close of his life the adulterous knight became a hermit, and died in the odour of sanctity.
Sir Lancelot is meant for a model of fidelity, bravery, frailty in love, and repentance Sir Galahad of chastity; Sir Gawain of courtesy Sir Kay of a rude, boastful knight; and sir Modred of treachery.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894