In Greek mythology, Gods were distinguished from ordinary people by being immortal. The Devil or Satan of the Semitic religions, whether fallen angel or not, must also be immortal. Were the Devil to die, there would be no one to tempt people into evil. Without Satan, the Semitic religions would not make sense. God and Satan is the duality of good and bad and both are needed to make the religion work.
Belief systems need dualities. Other examples could be ying and yang, light and dark, male and female, or trinities like Brahma, Vishnu, and Shiva, the Gods of creation, preservation and destruction in Hinduism. Competing ideas give richness and depth to mythologies.
Following Greek tradition (and who wouldn’t), Satan, being immortal, must also be a God, although a bad one. Archangels like Gabriel and Michael, and other lesser angels, would also be immortal and so also Gods. Are the Semitic religions beginning to sound more like polytheism than monotheism?
While science is a belief system with zero gods, religious systems can only have two or more Gods. Belief systems with one God are the only ones which won’t work.