A Vye is a shapeshifter, almost like a werewolf. The difference is that a Vye does not depend on the full moon to change into its wolf form, and more closely resembles a greyhound than a traditional wolf.
When in human form a Vye often had watery eyes. Every other part of their form will look like a normal human. It is mentioned several times that Vyes dislike their human form, for it causes harm ranging from itches to real pain.
They resemble a werewolf, only bigger and much more horrifying.
Vyes can be detected when asked puzzling questions or riddles because they will become agitated if they cannot solve them. However, they are very similair to humans in personality, having even the same desires as humans. In 'The fiend and the forge', Max has a run in with two elderly Vyes, who explain that Vyes are not profoundly evil, contradictory to what Max was tought at Rowan.
The ancestors of all Vyes were the brothers Romulus and Remus. They had been abandoned and left to die in the wilderness but were saved by a wolf. This wolf cared for them, and was no ordinary wolf, but rather an elder spirit in the form of a wolf. Some of the elder spirit's essence was passed onto the two brothers that she had raised. Remus later fell in love with a witch, and their babies would be the first Vyes. The fact that all Vyes can be traced back through their lineage to a witch, explains how Vyes are sometimes capable of performing feats of magic.
Vyes are shapeshifters; they can transform themselves to look like others, though taking a human form makes them uncomfortable. Vyes almost always work in pairs. One vye distracts its prey while the other sneaks up on it. If human skin is punctured by a vye's claws or teeth, the human has 72 hours before he or she becomes contaminated. Vyes hate bright light and the bitter cold, but they do not burn easily. They are very quick, but people can outrun them if they Amplify. Due to their ancestry, Vyes are capable of magic, albeit to a limited extent.