Fallen Angel appears to be born from angels that spend an extended time on earth and have gained an ego.
During the transformation and they shift from a holy attribute to a having a demonic attribute. However, fallen angels such as Dino have both, and thus have no particular weakness to either.
They have three pairs of jet black wings.
They were sent to earth to observe what was happening, and after a period of time, they gain an ego. Having an ego dramatically separates them from other angels in that other angels have no real sense of self or the ability to make their own decisions.