Sir Percival, or King Percival after Holy Grail, was a knight of Camelot, and was part of King Arthur's Knights of the Round Table. While searching for a set of legendary artefacts called the "Golden Boots of Arkanesses," he was kidnapped by Thorobshuun and Garagorshuun goblins in northern Asgarnia. He became their slave, and was thrust into a sack and forgotten for over a week. Percival is rescued during the Holy Grail quest and succeeds his father, The Fisher King as new king of the Fisher Realm. It is not known what happens to the Fisher King, though it is thought that he dies.
If you talk to Sir Percival after completing the quest in Goblin Village, you will ask "Aren't you supposed to be at the Grail Castle?", to which he will reply "You must have mistaken me with somebody else."
His name is a reference to the knight from Arthurian legend.