The Lancer-class Servant in the Fifth Holy Grail War of Fate/stay night.
The first hostile Servant to appear in this story, Lancer is a fierce but balanced warrior who enjoys combating a worthy opponent. He has a playful attitude and takes a very carefree approach to life, but is quick to work himself into a frenzy during a heated battle. Lancers are hailed as the "Most Agile" of Servant classes.
True Name is Cú Chulainn, the Child of Light, a hero from the Ulster Cycle of Irish mythology. He was a demigod, the offspring of a mortal woman, Deichtine, the younger sister of King Conchobar mac Nessa, and the deity, Lugh, the god who governed over the sun and a member of the Tuatha Dé Danann, the gods of Irish mythology. Deichtine, the daughter of a druid, Conchobar's father, and Maga, the widow of Ross the Red, disappeared in the Land of the Young without marrying anyone. There, she bore Sétanta, the child of the sun god.
In Fate/Stay Night:
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:
He is a Caster class servant.
In Fate/hollow Ataraxia:
Caren Hortensia acts as his Master during the events.