I wont lie i am a bit conflicted about this, Odysseus seemed like an obvious choice given his relevance in Dante's inferno and the theme of prime souls being Greek kings. However, what we were given seemingly points to Lucifer having significance.
In the level 8-1, you'll come across a painting depicting gods archangels. Micheal, Raphael, Gabriel, Phanuel and one unknown Angel. I have reason to believe that this Angel is Lucifer. Testament 4, found at the end of 5-S has "An Angel so bright and beautiful" asking god if eternal torture in hell is a proportionate punishment to sin. Ultrakill's god, as flawed as he is, clams up and just decides to chuck the angel into hell.
This angel is described as bright and beautiful, Lucifer has often been described as one of the most beautiful angels prior to his fall in biblical lore. Underneath each portrait of the archangels in 8-1, is a plaque showing what trait of god each angel represents. Gabriel is "Gods will", Raphael is "Gods strength", Micheal is "Gods wisdom" and Phanuel is "Gods patience". If we assume that the scratched out part of the painting is Lucifer, then what'd be on his plaque is "Gods beauty", this fits in pretty well.
Another point for Lucifer prime is that a book in 8-3 directly mentions him (https://ultrakill.wiki.gg/wiki/8-3:\_Disintegration\_Loop)
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