According to Arthurian History
What we know historically is that Sir Lancelot was almost certainly the creation of Chretien de Troyes, a French poet who lived in the 1100’s. Therefore it is safe to say that Lancelot was not a true historical figure. Some historians and writers have tried to tie the story of Lancelot to various companions or opponents to King Arthur, but even those elements are a stretch at best.
According to Arthurian Legend
According to Chretien de Troyes, who invented Sir Lancelot and included him in all four of his Arthurian romances, Sir Lancelot was a knight of the round table and a member of King Arthur’s Court. Chretien de Troyes assembled Lancelot really out of the best pieces of all the knights, skilled in arms and chivalry and courtly love and all the other qualities that make a knight successful.
In the beginning he is the King’s champion, fighting challengers and going on quests in the Arthur’s name. Lancelot is time and again the symbol of Arthur’s justice, and those who question Queen Guinevere’s fidelity are made to pay by fighting him. The irony is that in the end Lancelot would woo the Queen and they would become lovers. He is the most famous of the knights and the most accomplished. And yet, he is not the most perfect. That honor would fall to Sir Galahad, Lancelot’s son.
An interesting dichotomy is the fact that Lancelot is slandered by some for his adulterous relationship with Guinevere, while revered by others for fathering the perfect knight: Sir Galahad, even though Galahad was born out of a relationship with Elaine, with whom Lancelot was not married. (Some legends say they did marry, but the pregnancy was most certainly before their marriage.) And though Lancelot had taken the place of Sir Gawain in being the best knight-at-arms, Galahad would ultimately replace his father, Lancelot in being the best knight-in-spirit. As the legend writers turned to the Holy Grail, they incorporated the idea of purity of body and mind, and Lancelot did not fill the bill.
Galahad saw the Grail and realized the Grail Quest, while his the father was reduced to seeing an obscured vision and died knowing that his own sin caused his inability to see the cup of Christ. Ultimately, Sir Lancelot was the best Knight at arms but not in piety.