Archer's identity is that of an adult Shirō Emiya, the main character of the entire series. Emiya sacrificed himself so he became a hero like he always wanted. Archer is actually Emiya's future spirit form. Since the spirit world doesn't have a time line, the spirits can travel into any part of time and participate in the Holy Grail War.

and also the part where archer said Ilya, you're still as merciless as ever. That too also gave out some hints .

Ilya did looked shock when Archer said that

When Shirou thought he can be friends with Ilya, he is kidnapped to her castle. Saber, Rin, and Archer comes to save him together. While they were escaping, Berserker and Ilya stands in their way. Archer fights by himself to let the others escape. Saber, Shirou, and Rin gets to a rundown shack. By Rin's suggestion, they have a sexual intercourse to to replenish Saber's low mana. Having defeat Archer, Ilya and Berserker catches up to them. Saber fights Berserker equally, but she can't hurt his iron body. Rin did a surprise attack and blew up his head, but Berserker survived it. His Noble Phantasm was an immortal body that can regenerate itself 12 times. However, he is finally defeated when Shirou awakens to his true from of sorcery. Shirou creates a mirror copy of Saber's holy sword and defeats Berserker. Ilya had no more power to fight and Shirou lets her stay in his house. [source: Wikipedia]

Beserker's Noble Phantasm is his body. He has a total of twelve lives. So, he must be killed twelve times before he is defeated.

Berserker has been defeated like how many times.... 5 times already?

that was why Archer intended to keep on fighting Berserker eventhough he knows that he would lose.... right? so that it would be easier for the others to finish him

he died several times before killing Archer. Afterwards, Rin Tōsaka used her jewels to blast his head and killed him once by doing so. Shirō Emiya and Saber then used the Caliburn to finish off the rest of Berserker's lives.

Since archer canot finish the job, I think shiro himself is goin to take the last life out of berserker.

Archer's identity.

Archer lied to Rin when she asked on who he was in gamestory. The anime version makes Archer have amnesia remembering on who he was.

1. The pendant that Archer has is the key to who he was. The one which Rin used to heal Shirou in the fateful night. In the game, Archer gave the pendant but Shirou kept the pendant as Rin find outs in UBW. That confirms to Rin on who is her Servant is.

2. Shirou used to be in Archery Club and considered by club's president as a very formidable archer. Despite retired, she still wants to challenge Shirou for a final duel on who is a better archer. (game conversation)

3. The Tracing ability is what Rin shocked to discover is similar to Archer's Kanshou and Bakunya blade imaging.

4. Archer knew Rin's pecularity of tea taste. In anime episode 9, Rin asked Shirou to brew her particular tea. In gamestory, Rin was pleasantly surprised by Archer's tea making skill.

In UBW he plan to kill younger version of himself because he got sick and tired of the principle. When he sacrifice himself to be heroic spirit, he was summoned to be clean up crew in disasters, have to kill some to save lives. In the end he become so disillusioned that he decided to end it all by kill his own existence. Hence the Reality Marble of Archer has machine wheels at the background signifies his feelings that he is just an automata for specific purpose. The younger Shirou Reality Marble has no wheels in the background in contrast, symbolizing his belief in justice.