There are no such things as "fallen angels."
This includes the Christian idea of the devil, aka "lucifer."
No such thing.
There is only one G-d and no one can go against His will. (He gave humans free will, and thus allows us to make our own mistakes in the hope that we choose the blessing and not the curse (D'varim / Deuteronomy 11:26).
The word שָׂטָן / satan just means adversary. It is used in the Jewish bible to speak of humans as well as angels.
G-d has no adversary -- He is all powerful. There is only One G-d. Since angels have no free will the angelic satans do indeed "work" for G-d. They are adversaries of man and simply have a job to do (to tempt man to evil hoping that man will not turn to it).
Human satans, like all humans, have free will. Do you have any adversaries? Is there someone at work who wants to get promoted and so makes you look bad so they look good? This would be a satan -- an adversary to you.
Now keep in mind that human adversaries are OUR adversaries, but not G-d's. He is both all powerful and all knowing. . .
The idea of a devil (an evil counterpart to G-d) is called "dualism. The dictionary defines dualism as "the division of something conceptually into two opposed or contrasted aspects, or the state of being so divided."
In Christianity this dualism is personified by the good G-d and the bad god (devil). The bad god (devil) fights the good G-d -- this certainly describes the Christian concept of the devil. These ideas were common in the pagan world 2000 years ago -- particularly in Zoroastrianism -- an ancient religion which was practiced in ancient Rome.
From the BBC website regarding Zoroastrianism: "Dualism in Zoroastrianism is the existence of, yet complete separation of, good and evil. . . (there is an) ongoing battle between Good (Ahura Mazda) and Evil (Angra Mainyu) within the universe. It is important to understand that Angra Mainyu is not G-d's equal opposite. . ."
You may find demons, fallen angels and such in Jewish fables and stories -- but they are there just as wizards appear in Harry Potter books. They are fictions which are meant to both entertain and teach a moral story. They are not biblical, they are not part of Jewish teachings.
At this point many Christians ask "what about lucifer? He is found in Isaiah chapter 14!"
There is no lucifer at all.
The word appearing in Christian translations of Isaiah 14 is "lucifer."
It is a mistranslation.
The term in Y'shayahu / Isaiah 14 is הֵילֵל בֶּן־שָֽׁחַר -- transliterated as "heilél ben-shaḥar" or “Heilél" and it translates to "[the] morning star”. In modern English this would be the planet Venus.
How did the Christians turn the planet Venus into "lucifer"?
The Hebrew was translated into Latin.
In Latin lucem ferre means “light-bearer” -- which was the Roman name for the planet Venus.
It seems likely that the King James translators chose to shorten lucem ferre to "lucifer." Somehow over time the Christians associated "lucifer" (the mistranslation) with their "devil."
What of the nephillim?
Who were they?
It is a mistake to assume that נְפִלִים nephillim means"fallen", because grammatically that would have to be נְפוּלִים n'fulim; and the verb נפל (nafal) can just as well connote "to settle [in a region]" as "to fall" (see B'réshit / Genesis 25:18 and Shoftim / Judges 7:12).
The nephillim were also known as רְפָאִים R'fa'im, a name for the original population of כְּנָֽעַן / Canaan, both to the east and to the west of the Jordan. river.
Some of them (not all) are described as being very tall -- including עוֹג / Og (D'varim / Deuteronomy 3:11), and four brothers יִשְׁבִּי Yishbi / Ishbi (Sh'muél Beit / 2 Samuel 21:16), סַף Saph (Sh'muél Beit / 2 Samuel 21:18), גָּלְיַת הַגִּתִּי Golyat / Goliath of Gat (Sh'muél Beit / 2 Samuel 21:19) and לַחְמִי / Elhanan (Sh'muél Beit / 2 Samuel 21:19, Divrei Hayamim Alef 1 Chronicles 20:5). All these "giants" were the "sons of the רָפָה / Rafah."
Genetics -- tall men (very tall) from the same father.