Someone else mentioned earlier that once you became a Sheikah you would get white hair and red eyes, and I think other people have accepted the fact that once you become a Sheikah you will somehow get white hair and red eyes. I understand why people think this, merely because Impa, the only person we can be certain is a Sheikah has these. However, I think this is completely unrelated. For all we know Impa could just be an albino and therefore other Sheikah would have normal hair and eyes.
I also support the theory that it is possible to be a Sheikah, though i have just had another idea. Perhaps Sheikah's are like the shades from the inheritance series by Christopher Paulini (Eragon, Eldest, Brisngr) and occur only once every given amount of time, there can only be one in existence at once, or, it is very rare for there to be more than one. They are born with a certain mark denoting them as a Sheikah.
What I am trying to say is, perhaps Sheikah's aren't a species or a kind of super human, but an occurrence. Like a genius or say a disabled person. Something which occasionally happens making a hylian child a Sheikah.
