So, lets discuss what a Fury actually is.
Furies are pan-dimensional creatures that feed off Pain and Evil.
They are invisible and intangible in their natural state, and thus they can't feed. In order to do so, they set on the trail of pain and despair that evil men cause with their actions. They draw power from this pain, but it does not sustain them: pain and despair drawn from the environment charge their powers, so they can materialize in this world, in proximity of their prey. They consume then their victim, feeding off their Evil Soul and some body parts. Evil sustains them and allows them to live indefinitely, as long as they can find game.
Each kind of evil has its unique taste, so different Furies go after a different kind of evil person, from the meaningless school bully, to the guy who scratches the cars in the neighborhood, to the mass-murderer and so on... the only requirement to qualify as prey is to do evil intentionally, for gain or pleasure.
The Furies are in Black, with Silver and a bit of Lime/Neon Green. The basic idea is that while the Fury is fully visible, only the Silver is tangible, and you can cause damage only to the Neon Green (usually eyes/heart/core)
Furies are born from the souls of men, so they should be humanoid, but while younger furies are more human-like, and can be Dark Grey/Silver with a visible Neon Green Core, elder Furies can be bigger, less and less manlike (as their form degenerates in time), and only Neon Green eyes.