Cú Chulainn

A peerless great hero from the Celtic, Ulster mythology.
The "Child of Light" that received the blood of the sun god Lugh, his alias is hound of Culann.
He is sort of the same person as the Lancer that is summoned with identical True Name, but --
In both mind and body - he is somewhat more youthful than "that one".

(Source: Fate/Grand Order Wiki)