The path of a hero is never an easy one, and no one knows that quite like the Fate series' protagonist, Shirou Emiya. In Fate/Unlimited Blade Works, Shirou Emiya is thrown into the Holy Grail War by the skin of his teeth and has his ideals of saving everyone repeatedly questioned and (more often than not) smashed with a hammer.
But the one who criticizes his ideals the most is none other than Archer. A cynical bowman who somehow knows more about Shirou than anyone else, including himself.
The Fate series is no stranger to many twists and turns, but one of the biggest plot twists is this: Archer's real name is EMIYA, and just to be clear, EMIYA is not a descendent of Shirou or a clone. He's Shirou from the future.
Fate series' biggest twist
Any veteran Fate fan will say that when a newcomer starts the series, be prepared for some serious revelations. The idea that the red-garbed archer with a biting tongue, twin swords, a strange ability to replicate Noble Phantasms, and a ruthless 'do what I must' mindset is the sweet and naive Shirou Emiya's future version is one heck of a twist.
From the beginning, Archer has always been a sort of enigma among the Servant characters. Even a casual fan could make some kind of guess as to who each Servant really was. A long sword-using assassin, for example, wouldn't be too hard to peg as Sasaki Kojiro, even if he does give up his name in his introduction.
But Archer gives away no indication of who he is, his swords are Chinese, but his bow is western. Not to mention, he seems to be of Japanese descent. No archer in history matches that description, even in the Fate series' strange history (for example, Francis Drake in the Fate series may have been a woman or a queen in disguise). So, if the past doesn't hold the answer, why not the future?
The funny thing is, all Servants, when summoned, are pulled from a place called the Throne of Heroes, and those who accomplish remarkable feats in life are sent there. People like Hercules, King Arthur, and the like. Servants from alternate universes can be summoned, or heck, even storybook characters.
In the modern age, most people aren't worthy of becoming a hero, but Shirou Emiya, after the events of Fate/Stay Night's Holy Grail War, made a deal with the Planet (yes, that's a thing), in exchange for more power.
In return, the Planet would take him after he died, transporting his soul to the Throne of Heroes to serve as a Counter Guardian, a sort of clean-up crew for whenever something so catastrophic is about to happen. The Counter Guardians kill pretty much everyone in a five-mile radius; innocent or guilty, it doesn't matter.
Which is pretty much an endless hell for a man who just wanted to help people, regardless of who they were.
But, upon his death, Emiya's soul is now displaced from time, which means he could be summoned from across time/space, even to the past. So long as a summoner had something to summon him with, say, a special amulet, then it would be possible to summon Counter Guardian EMIYA.
Why is his name spelled in all caps? Mostly to differentiate himself from Shirou Emiya, the navie wannabe hero of justice. The Shirou Emiya of the series proper, however, is still very much learning, and when the future and past Emiya clash, it's a battle of ideals. And of Fates.