Born as Sètanta from a mortal woman, Deichtine, and the Sun God Lugh he's a Hero from the Ulster Cycle of Irish Mythology. He took the name Cú Chulainn after he killed the guard dog of the blacksmith Culann, said to be stronger than 10 men, when he was only a child. To make up to the old blacksmith he offered himself as a guard dog and since then was called Cú Chulainn, "Culann's savage dog".
Trained in the Arts of the Spear and magic Runes by Scáthach, he wields the cursed Gàe Bolg, the spear that inflicts wounds that never heal.
In Fate/Stay Night:
In Fate/Grand Order -First Order:
