He is most likely the First Born Son of Gwyn. The Storm Curved Sword states, "When the great beast fell, the king claimed his soul, as was the custom in the age of gods." so we know he is at least as old as Dark Souls 1 and his other item the Dragonslayer Swordspear states, "The swordspear is imbued with lightning, of which he was the heir.". Thus being at least as old as the Age of the Gods and the heir to lightning(Sunlight) there leaves little room as to who he is if he is a character already established in lore. Speculation: All this information coupled with the similarity in appearance, height, build, and fighting techniques has led many to conclude that he is without a doubt Gwyn's longlost heir and perhaps was expunged from history to hide his "shameful defection" from his father's kingdom in favor of the everlasting dragons.