An Angel is a loyal Heavenly creation of God who dedicates their life to serving the Lord. Namely the Omniverse God. However an Angel can also be an ascended Human who attained Angelhood after dying. An ascension that continues to their own Godhood in the infinite higher dimensions of Heaven. Non-Human Angels were specifically created by God directly as his Heavenly sons, and daughters who in essence have a perpetual childhood, and live only to serve his Heavenly Kingdom. Human Angels were meant to grow into equal Gods themselves since the Omniverse God was once a biological being himself from another Omniverse.
A Fallen Angel is one who falls from Heavenly grace by placing something else first in their mind above God. Generally something in the physical Universe where they end up manifesting themselves in a biological form. Or it can just be themselves if a big ego develops, and they lust for power of their own. This is how the first Fallen Angel, and Demon, the ArchDemon Devil, was formed. Some of the ancient Earthly gods were in fact Fallen Angels as well. The Fallen Angel’s DNA lies somewhere in between that of an Angel, and Demon. This Angel-Demon Hybrid state makes them stronger than even an Angel in biological form, and most certainly a Demon as well. Luckily the Fallen Angel is a temporary creature as they are a transitional being headed for Demonhood. If they don’t bring themselves back to being one with God they will be damned as a Demon for all eternity.
A Demon is an Angel who has not only left God behind but partakes of evil deeds in his quest for power, allegiance to the Devil, or simply for enjoyment. A Demon has nothing but contempt for God, the Angels, and all his creations including humans. In fact they hate humans most of all out of jealously since they are destined to ascend to Godhood. Once an Angel transitions into a complete Demon they gain a sub-conscious loyalty to the Devil as they once had for God. It’s automatically programmed into their minds. Although there are many unholy temptations for an Angel to break his sire bond with God there are very few equivalent holy temptations that would allow for a Demon to return to Angelhood. However it’s theoretically possible.