The furies are spirits of anger. As such they should not be considered Fey (which are spirits of Nature, unlike what most humans think, anger is not a fundamental force of nature but is crafted entirely by sentient beings on their own). In fact the furies are more the manifestations of anger than actual spirits and they are called into existance by acts of (usually unfounded or unjustified) rage.
Furies have the ability to meld with sentient beings, giving them the power of rage. As such they are much sought after by those seeking war and glory. Usually this rather dubious honour is reserved to those whose anger called the furies, however a triplet of furies have taken residence in Leaf, saying his anger is pure. The meaning of this or what it may mean for poor Leaf is as yet unknown, however it has become appearant that when Leaf gets angry about something the fury magic stirs and is unleashed (usually resulting in Leaf being drained for a while and fainting). So far this has happened once (and nearly a second time in Gamun), and Leaf and his friends are worried about what might happen the next time.