Belial is a compound word, believed to have been taken from the Hebrew beliy, meaning “not,” and ya'al, meaning “profit” or “benefit.” It is used twenty-six times in the Old Testament, usually translated as “worthless” in the New American Standard Bible, but also as “base,” “destruction,” “rascally,” and “wicked.” In the earlier books of the Old Testament, when describing a wicked person, the King James sometimes uses “son of Belial” (or “daughter,” “man,” or “people” of Belial). Like many other uses of the term son of, the expression “son of Belial” doesn’t imply that Belial is a real person who fathers children; rather, it’s a description of people characterized by worthlessness or corruption.
A less accepted theory is that belial comes from beli 'ol, meaning “without yoke”; the definition would remain about the same, however: a “son of Belial” would be someone who is “lawless” or “rebellious.”
Either way, the Old Testament uses Belial as a personification of evil, not an actual entity. Later, the personification began to be thought of as an actual person, and in the New Testament Belial is used as a proper name of Satan in 2 Corinthians 6:15.
In Jewish Apocryphal literature, Belial is often presented as an actual being and given a back story. Several Hasidic books, including the Book of Jubilees, say Belial was one of the angels who followed Satan in his fall. The Sibylline oracles (supposed prophecies recorded by the Sibyls between AD 100 and 500) say Belial will be the Antichrist and/or a fake Messiah who deceives Samaritans. The former claim may be influenced by 2 Thessalonians 2:3, which calls the Antichrist the “man of lawlessness,” but the oracle is also associated with Nero.
Belial as a demon has been adopted by more recent literature. In Paradise Lost, John Milton writes that Belial was the last demon to fall and the vilest—the demon of impurity and lies. In some occasions that Belial is used as a personification in the Bible (Judges 19:22; 1 Samuel 2:12), Milton directly attributes the evil to the individual demon. Belial is also mentioned in the Lesser Key of Solomon, a spell book that promised the user power to control demons for personal use. The book has nothing to do with King Solomon, as it was written in 1600s. In the last century, Belial showed up as a character in various novels and video games.
In Scripture, Belial was used to personify wickedness and worthlessness. The only time the word is used to identify a person is 2 Corinthians 6:15, where the name is applied to Satan. There is no indication from Scripture that Belial is the proper name of a specific demon.