Where ghouls are different enough to be seen as a whole other sort of humans, Grayfells are a horrible case in-between the two. They can be found across the entire American Wasteland, but their preferred habitats, isolated ruins and irradiated swamps, can make it hard to find some of them.
The reason why Grayfells aren't as common as the typical ghouls, is unknown. It seems that not everyone is capable of turning into one when exposed to radiation, and the percentage is in fact very low. The process that turns a person into a Grayfell resembles in much the ghoulification process, but it seems to stop somewhere halfway. The amount of radiation needed has to be quite exact, and the same goes for the exposure time. Too much or too long, and you're almost sure to turn into a ghoul. Not enough rads, and you'll get away with some radiation sickness (if lucky).
The name 'Grayfell' wasn't randomly chosen: they look like corpses with their chalk-white/yellowish skin like the decomposing flesh of a dead beast. Their spine and other bones start growing crooked in unnatural and painful angles. Their condition resembles lepracy in everal ways; hands and feet often tend to shrink and fall of when they're not treated properly, meaning that a lot of them live with parts missing.
Grayfells eat what most normal people eat: meat from wasteland animals, vegetables and leftovers from the Old World. They have a hardy constitution, meaning they are capable of drinking irradiated surface water without too much side-effects. This comes in handy, as most of the bodies of irradiated water are unsettled and they won't get too many visitors.
Perhaps the most important thing in which Grayfells are different than ghouls and Super Mutants, is that the fact that some of them do not turn completely sterile and are still able to produce offspring. These children are always born with the same condition as their parents, but at a degree worse. Only few of them make it to adulthood. In that fase, they can hardly be called human anymore. Their severely deformed bone structure, rotting skin and bulging eyes make them look like grotesque monstrosities. This, among other reasons, is why Grayfells live in isolated communes far away from civilized regions. They can still use guns and other weaponry, like spears, to defend themselves, but hardly anyone ever finds these small settlements in the vast emptiness of the wastes. Their only real enemies are predators like the Bloaters in their swamps and the Harbingers in the sky.