Speaking from experience, though the cataract itself did not affect my eye color, I do have differently colored eyes as a result of the surgery to remove the cataract.
I'm pretty sure the tags mean Aura drew Natsume. The minor character. Unless Natsume is a moe rule 63 on Aura, the two are separate. Aura's penchant for heterochromia suggests that Natsume was created especially for him.
Aura said it's rheumatoid arthritis, and that she's half blind on one eye. Wikipedia tells me that probably means her joints are fucked up, and it can mess up other organs as well, such as drying up the eye.
Her body looks pretty healthy and not-gnarled for a person with that disease, but I assume she's in an early stage or something.
I'm not a doctor, I have no idea if it's possible for the iris to change colour from that kind of damage. I don't know if it's impossible either, we'll have to look it up with one of the KS medical guys.
CID 20766, posted on August 25, 2010: