Jigoku wasn't named Jigoku before, infact, his name was Simon, he changed it to Jigoku due to personal reasons.

Jigoku's name is not a funny Dragon Ball reference, Jigoku's name in japanese means Hell.

He's not the guy to fuck around with.

Jigoku is a cancer patient, however, his cancer only boosts his regeneration factors like deadpool, his cells are replaced by cancer cells everytime he loses a limb.

Jigoku's Retronian Hunter form has inspirations from Ramamba from Garten of Banban 0 and a very rare and lethal condition.

Cyclopia (search it at your own risk) is a rare form of holoprosencephaly and is a congenital disorder (birth defect) characterised by the failure of the embryonic prosencephalon to properly divide the orbits of the eye into two cavities. It is the severest facial expression of the holoprosencephaly syndrome.

In other words: a baby is born with 1 eye hole but both his eyes in the eyehole.

Jigoku does have that condition as a Retronian, but rather than that, it works like a big eyeball, literally a fairytale cyclops but more gore-ry

Jigoku is the creator and starter of Project Reborn, Igor and Katherine were just the lead scientists, they would manage everything instead of Jigoku.

Matthew has been abused by Jigoku, but he can't tell his parents, or else Jigoku would use him for his experiments, and actually.

Jigoku was the one that gave Matthew that trauma, and that big scar exposing his skull.