Though he was the supposed half-nephew of King Arthur, Sir Constantine was one of the less prominent Knights of the Round Table. His father was Sir Cador, the King of Cornwall.

Along with Sir Baudwin, Arthur entrusted him with the joint-regency of Britain when he left to fight his continental war against Emperor Lucius of Rome. Constantine was therefore in position to receive the throne when Arthur died without an heir. As one of the few survivors of the battle of Camlann, the King gave him the crown of Britain on his deathbed. Sir Constantine was a good king, who restored order to the realm and the Archbishop of Canterbury to his diocese. He faced many problems with the invading Saxons and with the two rebellious sons of the wicked Sir Mordred, but was able to overcome them. When Mordred's sons took refuge in churches in Winchester and London, Constantine pursued them and, disguising himself as a Bishop, killed them both before the high-altars. Constantine tried unsuccessfully to persuade Sirs Bors, Ector de Maris, Blamore, Bleoberis and other former companions to remain with him in England after Lancelot's death and to keep up the Order of the Round Table; but the flower of Arthur's knighthood had either been slaughtered, moved to Gaul or entered the solitary life of a hermit.

He should be identified with the historical King Constantine of Dumnonia (Cornwall/Devon/Somerset) who lived in the late 6th century. He was the son of King Cado and is best known for having been rebuked by St. Gildas for his wicked ways, though he later became revered as a saint!