For some reason my gut's telling me that this is Omegon/Omegan, but that's because I've always thought of Alpharius as this 3srs5business sort of character. Not sure why, it's just stuck in my head.
Something else has occurred to me- what if their hair isn't actually a sidecut/undercut? What if they actually have a short strip of hair cut into a mohawk style, allowing them to switch the sides the hair falls on when needed? Depending on the time of day/the needs of the plan/whether or not Tzeentch has had berries in his breakfast cereal/the lulz, this might be Schrödinger's Primarch, in that they're both Alpharius/Alpharia and Omegon/Omegan at the same time until the wavelength collapses.
Pretty sure this is Alpharius. Head of the Hydra states clearly that Alpharius and Omegon swapped identities the moment the latter was found because Alpharius was instructed to be the secret word of the Imperium, whereas Omegon craved recognition more and wanted to make his presence known to the rest of the Primarchs, so Omegon takes on Alpharius' identity as the public face of the Legion, and Alpharius "becomes" Omegon to keep performing the role he was groomed in.
That is, until Omegon gets killed by Dorn and Alpharius takes back his identity.
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