Your character could be a enemy that changed his or her appearance and became a "good" guy for some reasons. The enemy could be setting the PCs up, or could be trying to change its stripes. Go to Comment
Real World: some Indians in the Amazon treat their eyes with a traditional potion applied with palm leaves. Brutally painful, the drug alters vision, giving the jungle's dense green walls greater texture and dimension. You could adapt this to desert or swamps, or other hard to navigate regions.