Saber is the Saber-class Servant of Kiritsugu Emiya during the Fourth Holy Grail War, and then of his son, Shirou Emiya, during the Fifth Holy Grail War.
Her True Name is Artoria Pendragon, better known as King Arthur, the “Once and Future King” and a legendary hero of Britain. She became king by pulling the sword Caliburn from the stone. Later, the magic sword was destroyed, after which the Lady of Lake gave her Excalibur and Avalon, becoming the most notable items in her possession.