Darn, I came up with the same answer. Didn't post fast enough.
I'll post my reasoning, but mark it with spoilers while we wait four battousai's answer.
(SPOILER)The riddle asks for "the only pure ones in the game." From some of the various clues and hints I took this to exclude characters who are not 100% human, often because of major invasive medical/surgical procedures. We can rule out anyone who's had JENOVA experimentation/Mako treatment because of the JENOVA cells/Mako energy in their bodies (i.e. Sephiroth, Cloud, Red XIII, Zack, Lucrecia). We can rule out Barrett because of his mechanical arm, Aeris because she was briefly experimented on (also she is an Ancient and arguably not of the human race), and Cait Sith because he is a...whatever. That leaves Tifa, Yuffie and Cid. Tifa is arguable if one assumes that she has had surgical breast augmentation (thus meaning that about 35% of her body mass comes from silicone).
