Fallen Angel:

How is the fallen angel characterized?

Extra-biblical sources say that the ‘fallen angel’ [singular] was the highest ranking rebellious angel that was cast out of heaven.

This teaching is not in the Bible.

Note: It is not stated or even implied anywhere in the Bible that a fallen angel was cast out of heaven.  Revelation 12:9 does not state a fallen angel. And the great dragon was thrown down, the serpent of old who is called the devil and Satan, who deceives the whole world; he was thrown down to the earth, and his angels were thrown down with him.

A fallen angel cast out of heaven is a myth.

A fallen angel is a myth.