crack theory of the day: Cain is an aspect of Fulgrim, a shard of his soul or humanity or whatever, a remenant. Like, the sword skills, the quick friends he makes with a (suspected) Iron Hands successor chapter, the Slaaneshi claiming he could easily be one of them, the constant shit talking he does of their outfits & color choices...
the constant imposter syndrom & anxiety about how he appears to others (because he failed, he was never good enough, he could never live up to the expectations). the ludicrous amounts of luck that really make no sense unless he's being actively favored by something (like say Big E). and no one knows cause Jurgen is right there 99% of the time & whatever remains that is identifiably Fulgrim only really truly shines through in situations where Jurgen is around, and since he's a blank that kinda blocks any sort of residue or whatever that could clue people in