Heroic Spirit of the bow. A type of Servant that has low basic parameters, but makes up for it with Skills and Noble Phantasms.
Of course, the Archer in the story was no exception, fighting to the bitter end while freely making use of unknown Noble Phantasms.
He is a thoroughly pragmatic cynic, but oddly enough, he also has a childish streak to him that makes him impossible to truly dislike.
His true identity is XXX. He is important enough to be considered a shadow protagonist, and it could be said that half of Fate/stay night is his story.
At the beginning of the game, his memory was supposedly fuzzy due to an imperfect summoning, but in reality that was only partially true.
The night he was summoned (after Rin went to sleep), he assessed the present situation, figured out his circumstances, and concluded that the opportunity to complete his objective had at long last appeared.
However, he didn't realize the girl who summoned him was Tohsaka Rin until she introduced herself. (His memory wasn't fuzzy, the name “Tohsaka Rin” had simply been worn away by time.)
At that moment, it all came rushing back like lightning.

“Then I'll call you Rin. …Yes, it suits you well.”

Archer's murmur at that time was thick with a heartfelt and madly passionate affection.
…By the way, there was lots of vague foreshadowing about Archer's identity throughout the game, but none was more subtle than after the battle with Berserker in the Saber route.
Also, his skin was darkened from the rebound caused by using a certain spell… well, that's the story, anyway.

Fate/side material: Fate Encyclopedia