The Mirror Reaper is Odysseus/Ulysses
In the Divine Comedy Odysseus and Diomedes are both trapped as twin flames in the 8th bolgia, the one to punish False Counselors; I think this was referenced in Ultrakill in the form of the Mirror Reaper, a being formed of three human bodies, one Ulysses, one Diomedes, with a unknown third, possible Laocoön or Cassandra, who both warned the Trojans about the horse, but weren't listened to, although they wouldn't fit as "False Counselors". This theory is mostly fueled by Odysseus being mentioned in the books in 8-2 and 8-3, while also not appearing as a corpse in the layer. Anyway, this theory could be completely tore apart if Ulysses is revealed to be the third prime. Make sure to comment your opinion on the matter, and remember, this is just a theory, A GAME THEORY!