The unfortunate foul-bloods.
The fay-blood's existence is due to an unholy union between a fay and a kykr. If there's a couple of generations back when this happened then perhaps they are fortunate enough that their fay-bloodedness does not openly show on them. But if it's within one or two generations, it is generally hard to hide that they have fay blood in them.
Depending on how strong the fay-blood is, it manifests in different ways. Among those who have a fay ancestor 3-4 generations back there are usually few if any visible marks of their condition. They instead only perceive the fay better and can usually see through fay illusions better than others. If it's within a generation, they usually get a very strong connection to the wilderness and find being in the open wilderness to be a liberating experience. They may have some signs of their ancestry, such as very dark, even black, irises, scaly or bark-like spots on their skin or a glittering light in their eyes. Like those with weaker blood they also have an increased sensitivity for other fay and their magics. Those who are unfortunate enough to be the direct child of one fay and one kykr, they are truly half-fay and as such it's almost completely impossible to hide the condition. Depending which species of fay their parent was, they will have completely black eyes with no sclera, bark-like coverings across their back, neck and shoulders and sometimes atop their arms. They can have troll-like tusks and even small horns on their forehead. These half-fay have fay blood strong enough to actually link with and make use of fay magic. For this reason alone, they are feared and hated among the kykr. The final form of fay-bloodedness is to have the blood of the Inuelweri, the royal caste of fay, in them. These fay bloods usually have a slightly easier time to hide their ailment as the inu themselves look similar to the vindral, so the differences aren't that outstanding. Their skin usually takes on a blue hue and their eyes lose contrast, making the pupil and the iris harder to see, as their eyes become an almost uniform blueish white hue. However, that is not all. They also leave a faint smell of spring rain and forests around them, and they have a habit of walking very softly and gracefully on their toes, having to concentrate hard to suppress that instinct in order to blend in. It's also not unusual for their hair to be very long and waft, as if in a gentle breeze, even when indoors and there is no wind to be found. These fay-bloods have the strongest connection to the fay and can sense, identify and even many times break through fay illusions and seem them for what they really are. They can learn and master fay magic on their own, and can even tell if fay magic has been cast on a person or place and with some effort, even identify the spell used.
Fay-bloodedness is hereditary and incurable. Those who suffer from it are born with it and they will die with it. There is no way to cure it.
Those who are fay-blooded are seen with suspicion, distrust and sometimes open hatred. It depends on how obvious their condition is, and how they behave. Someone who is reclusive and backs down from conflict might be bullied and get spat at on an open street, while those who are proud, strong and confrontational often find themselves at odds with armed guards.