Lancer

Heroic Spirit of the lance. Requires supreme agility, as well as excellent close combat skills. Though they tend to be less flashy than the other classes, they are a very reliable type of Servant.
The Lancer in the story has a rough and tumble personality, but at his core he believes in justice and is easy to get along with. He is an agreeable youth acting out the role of one gentle in disposition yet strong in body.
His true identity is the great Celtic hero, Cu Chulainn.
If the stage of this Holy Grail War were anywhere but Japan, he would be a shining existence on par with Heracles and King Arthur.
As was mentioned in the story, he is a Heroic Spirit that specializes in survival, capable of returning alive no matter how hopeless the situation. Unfortunately, this time luck wasn't on his side.
It is said that in the midst of battle he would unleash a fearsome war cry, and at the same time his body would swell and bulge with muscle as his behavior became like that of a demon.
However, though he possesses the qualifications to be a Berserker there is no doubt that the Lancer class suits him best.
He responded to the summoning solely because he wanted to engage in a valiant fight to the death, but this simple wish was never granted to him.
He is a tragic Servant, who embodies the cynical notion that “wishes don't come true.”

Fate/side material: Fate Encyclopedia