Belial is the one name in the catalogue with a continuous documentary history running back to the Hebrew Bible, and the history is not quite what the grimoire assumes.
In biblical Hebrew beliyya'al is not a personal name. It is a noun, of disputed etymology, often read as without worth, used in constructions such as sons of Belial or a man of Belial, meaning a worthless or lawless person. English versions from the Authorised Version onward transliterate rather than translate it, which is why generations of English readers have met it looking like a proper name when in the Hebrew it is not.
The personification is later and is documented. In the Dead Sea Scrolls Belial is a figure: the prince of the dominion of wickedness, opposed to the Prince of Light, commanding the spirits of his lot, in a dualism that structures the War Scroll and the Community Rule. That is the point at which an abstraction becomes an actor. 2 Corinthians asks what concord Christ has with Belial, treating the name as the opposing principle. From there the figure passes into Christian demonology and appears widely in the medieval and early modern literature.
The Goetia inherits the end of that chain. Its Belial appears as two beautiful angels in a chariot of fire, speaks in a fair voice, distributes preferments and senatorships, and requires offerings before he will answer truly. He is also, in the entry, said to have been created next after Lucifer.
The transit from a Hebrew noun to a courtly patronage broker in an English manuscript takes roughly two thousand years and passes through at least four languages. Very little of it is guesswork, which makes this entry the best-documented case in the book.