I don’t know if I’m the novelisation, there is a big reveal in who he really is? The origin of his immortality and so on. This season reveals the origin of his memory loss, which is my favourite part of this drama.

I’ve had my thoughts on his possible real origins

In my feeble deductions, I’ve pondered the meaning of his name ‘heartless’, his missing heart and his immortality.

Is he the Monkey King? The monkey king wreaked havocin the heavens and was cast out.

Is he the Monkey King, cast onto  Earth, wandering for eternity, earning his way back to the ascended plane? Thelack of a heart, I feel supports this theory.

He’s been wandering for so long, he no longer has certain memories anyway, even before he’s bestowed the gift of forgetting, in season 3. I’m thinking he was cast down, CURSED for all his mischief and wrongdoings, unable to forget, as such in eternal torment, with only purpose to seemingly seek his death and unbeknownst to him, his urge to help people is part of his redemption arc. 

 darvng:

It's never addressed in either the book or the series so I guess it's entirely up to your interpretation. But I've seen theories saying he's from another planet, or that he's a zombie-like creature whose spiritual energy (?) was lost, and also some saying he's an incarnation of Sun Wukong. So you might not be that far off after all lol

It feels like it would reduce this story a little, if he was just an alien.
I guess he is definition of undead, as he's reanimated, has no heart ,if we put aside requirements of zombies, who tend to be into eating the living.
It makes sense if her was at least part of Wukong's soul, as they have similar abilities, like True Sight, truly insane immortality, Taoists, is name Wuxin - ''heartless'' and Wukong "awakened to emptiness". In some religions he has a heart of stone, which could explain Wuxin's missing heart.
Wukong can live without his heart.

It would at least make sense, and be yet another cool Wukong story.