There were several important characters. Sir Percival was a key figure. He had a journey of self - discovery during the Grail search. Guinevere, Arthur's queen, also played a role as her relationship with Lancelot affected the court and the knights' morale. And of course, the Grail itself was like a character in a way, as it was the object of the great quest that drove the story forward.
Arthur himself was very important as he led the knights on the Grail quest. Then there was Sir Galahad, who was seen as one of the purest knights and was fated to achieve the Grail. Also, Merlin, the wizard, was important as his magic and wisdom influenced Arthur's court.