Cú Chulainn

A Lancer-class Servant in Nero’s army.
The Heroic Spirit of a figure in Celtic mythology from the Ulster province of Ireland. His father was Lugh, the god of light, which earned him the nickname “Son of Light.”
His childhood name was Setanta. As a young boy, he killed the beloved guard dog of the smith Culann in self-defense, and promised to rear a replacement to make up for the incident. Until the dog was old enough to fulfill its duty, Setanta vowed to act as Culann’s guard dog, earning him the name Cu Chulainn, which means “Culann’s Hound.”
In Fate/EXTRA, he fought the Main Character as the Servant of Rin Tohsaka, and was summoned again to the new SE.RA.PH in recognition of his important role in the Holy Grail War.
Although he was originally the Main Character’s enemy, he showed more friendliness than animosity once the conflict between them ended. He respects both the Main Character and Nero, and is serious about helping Nero’s army protect both the new SE.RA.PH and humanity on Earth.
A long-time veteran of SE.RA.PH’s Holy Grail Wars, he is acquainted with Archimedes from past encounters. Although ultimately they operate in different ways — Cu Chulainn being something of a jock-type to Archimedes’ studious-type — the two have a good relationship. As a warrior, Cu Chulainn recognizes and respects Archimedes’ ruthless side. Due to the nature of their relationship, if he were to discover that Archimedes had plotted against humanity, Cu Chulainn would accept it with a passing remark like, “Oh… yeah. He’d probably do something like that.”
He only joined Nero’s army because it “seemed interesting,” and was not expecting to encounter Nameless, who had become a general under everyone’s nose. “Ah, crap,” he scowled.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA