[quote=GuyYouWishYouDidntKnow]
What Tatsua's saying is that a Servant needs some historical/mythological grounding, they can't be entirely OCs. EMIYA was eligible because of Counter Guardian shenanigans, Rin's pendant, and things of that nature. Fake Assassin has a historical basis but never actually existed, so the Grail searched through the Akashic Record for a wraith most similar to the legend of Sasaki Kojiro and summoned that wraith, which only ever happened because he didn't have a real summoning. The point is that both EMIYA and Not!Kojiro had justifications for being summoned as Servants, completely "original" Servants don't work in the same way. They need to have existed historically or in legend, rather than being some legend you made up.Also Mjolnir doesn't work as his NP for a couple other reasons, but that comes secondary.
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I had no idea how wonderful it felt to have somebody with a similar suite of knowledge knowledge in a sea of people without it. Until now. I love you~

As this awesome fellow said, EMIYA is a complete anomaly resulting from a clusterfuck of extraordinary circumstances all coming together at a precise, unlikely moment during the fifth HF. Using him as an example isn't fair, he isn't in the same category as ordinary Servants to begin with.