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

Spoiler

Originally the servant of Bazett Fraga McRemitz, he becomes the Servant of Kirei Kotomine after Kirei fatally wounds Bazett and remains under his command for the duration of the War.

In Fate/Grand Order -First Order

Spoiler

He is a Caster-class servant. Unlike the other Servants, he's not blackened and aids Mash and Ritsuka in their task

In Fate/hollow Ataraxia

Spoiler

Caren Hortensia acts as his Master.