I don't even remember any mention of a transplant... that might just be my memory, I'm juggling way too many games, but I only remember mentions of how his face got scared and his eyes fucked up by whatever mysterious incident happened, not anything about a transplant...
The Transplant Part is my wild guess
Remember The mc evolution to stage 2 ?
the one voice told mc not bend knee for the 1st
that voice use symbol The eye
sooo maybe mc's eye indeed special one
Huh, never thought about it from that angle before... maybe he indeed has been carrying a monster part with all these years and that's because he managed to attune so well to the transformation. His father seems to be pretty involved with the underworld after all so it wouldn't be a too much of a stretch for him to have acquired a monster part during his "job", and being desperate enough to implant it into his son if his life was at stake. Don't think dad is a monster though, maybe the mom got monsterfied in that incident where she "died" but is probably still roaming... maybe she is that black monster that shows in the bad ending when Ella tries to offer the MC to the 4th?
My wild guess ?
Mc mom killed by the someone got infected from either the 3rd or 4th ( the one MC didn't Choose )
then MC dad killed the infected
and knowing MC lost his eye decided took the infected eye and transplated to mc
or for better twist and more tragic
MC mom got infected from either 3rd or 4th
but failed become berserk like kenny and hurt MC too
MC dad want stop infected Mom and kill her
he took the eye and transplated to MC
afterall MC father always dodgy about truth Mc mom death
I wonder What MC father job is ?
Maybe Monster hunter ?
he not have superpower
but have power of prep like michael before got infected
it will badass is that true
And about the black monster in the dead end
Maybe that thing Is the big bad the one mommy and daddy want to kill
But That thing too strong Crushing mommy like bug
that why She need army and secret weapon to kill black monster