Not Schizo, hmmm..

Okay I'll wait for a few more guesses to come up I think, though DelightfulSpekkio442 gave a close explanation I might as well give the full solution to my last one while I think.

Two stand alone, or maybe three, can't tell if thats a fake you see.
For if it is, she moves damn quick, ignore that though, stay on topic. (This is a hint to the form of the answer, it's also referring to Yuffie's leg)
The question here asks you to name, the only pure ones in the game. (The only leap is that it is talking about party characters, which I hinted at later)
Though thinking back you may suspect, that of the two one's not correct.
But lets ignore the chance that she, has had cosmetic surgery. (This is indeed talking about Tifa)

Suppose we may have started wrong, beginning with the three.
Let us for now instead discuss, the ones it cannot be.
Those in the forces they're straight out, you see it in there eyes. (This is talking about Cloud and Sephiroth, due to their Jenova history)
We cannot count the tested ones, now in their roots it lies. (This actually talks about Nanaki and Aeris, due to their Hojo encounters, it is their family background however that rules them out)
A few more left to be ignored, but I'll leave those out for now.
For you can work them out you see, the question though is how? (This is talking about Vincent, Barret, and Cait Sith, who are to be worked out from the rest)