Other worldly demon/oni types often seem to get the red eyes... Nerine in Shuffle! or Etona in Disgaea.
Why the humans don't notice hers... I could rationalize my heart away but basically its a way to denote her as unhuman. I guess people see only what they want to see
(I keep borrowing quotes from The Point lately).
I suppose I'd argue that her eyes may come off as a bit oddly colored but not totally alien. I've known people with hazel eyes (which just means lots of colors) who had a fair amount of red mixed in with the yellows, browns, and greens in their eyes. My eye colors (hazel) have changed over time... they used to have more red but are now a bit more green.