Ashborn is a Monarch, to be specific, the Monarch of death.
Monarchs cannot manifest into the human world without a vessel, that is why the leveling system jinwoo uses exists. The Architect (creepy smiling statue in the beginning of the story) devised the system to gradually train/change the MC body for Ashbor to use as a vessel.
Using Jinwoo as a vessel would also mean that Jinwoo consciousness would vanish.
No, the father is not a Monarch. He was left inside a dungeon that was closing and if I remember correctly he died there.
He was then resurrected for a limited time by another party, which I won't mention otherwise I would spoil the ending.
If my memory is good, since I read the novel at least 1 year ago... He was sent to earth to kill the monarch of death, not knowing is his son.
He will not kill him btw, but I don't want to spoil anymore than this, is one of my favourite scenes of the story.
Just to be clear, if we have to stay faithful to the official novel translation...
-Kings: refers to extremely powerful hunters
-Nation Level Hunters: in the novel are hunters that cleared a S dungeon (like Jeju), in the manga for now, they seem to be the ones that cleared the dungeon of Kamish
-Monarch: a God-Like being who comes from another dimension to conquer earth. Every Monarch controls a type of monster. Also, a monarch can create a gate.
-Rulers: ending spoiler material, I won't spoil that.