Fallen Angels are Angels that have fallen from the grace of God, due to having "impure thoughts" that divert them from the teachings of the God in the Bible. The leaders of Grigori were tempted by human women and had fallen after having sex with them.
Despite being cast out of Heaven, Fallen Angels and their subordinates have been known to occupy churches and use them as bases. However, they go out of their way to destroy all of the religious icons in these churches in the process.
Currently, the Fallen Angels have the smallest population among the Three Factions.
Fallen Angels have an appearance similar to their Angel counterparts with their wings; however, their wings are colored black as opposed to the Angels' white wings. Also, because they have been cast out of Heaven by God, they also lack the halos above their heads. Fallen Angels are often seen wearing dark-colored clothes, mainly pertaining to the colors black and violet.
Like their Angel counterparts, the rank of a Fallen Angel depends on how many wings he/she has. For example, Azazel, the leader of the Fallen Angels, has six sets of wings which justify his position. The blackness of the wings of each Fallen Angel is also different, Azazel, for example, has jet-black wings with feathers that have the color of everlasting darkness and is darker than endless darkness.
Although they have been cast out from Heaven, Fallen Angels retain the ability to use light-based weapons. This type of weaponry is extremely harmful and poisonous to Devils, as it is shown many times when Issei is wounded by Dohnaseek and Raynare.
Unlike Angels and Devils who require the use of a respective Brave Saint or Evil Piece to replenish their numbers, Fallen Angels can replenish their number of "pure" Angels simply by tempting the remaining pure Angels in Heaven to join them. It is unknown if it works in the opposite direction, where the Fallen Angels can redeem themselves via selfless and righteous acts to rejoin Heaven. They can also increase their numbers through childbirth, as with the case of Baraqiel with Akeno through Akeno's mother.