Who exactly is Archer from the Unlimited Blade Works movie? Although I have not watched the Fate/Zero TV series, I have watched the Fate/stay night series and the Fate/Stay Night - Unlimited Blade Works movie. At the end of the movie I found out that:


   Archer is actually Shirō. Even before the ending, Archer told him he has already experienced what Shirō goes through. So I did not know if he is actually Shirō from the future or if he is a completely different character than the one in the TV series.
The Archer-class Servant in the fifth Fuyuki Grail War is: 


   Shirou Emiya from an alternate timeline than that of the Fate/stay night timeline. The Shirou from the alternate timeline made a contract with the world and became the Heroic Spirit EMIYA.  
 He is not a proper Heroic Spirit in the sense of other heroes (e.g. Saber, Lancer, Berserker). He acts as a Counter Guardian for the world as payment for the contract. 
 He is summoned because he still has the jewel pendant that Rin used to save his life after Lancer stabbed his heart. The jewel he owns is the actual catalyst used for his summoning rather than the one that Rin still has at the time of his summoning.