You can´t die, if you are already dead.

A person has died, he does not know how or why, but after his death he meets a girl dressed in white. She is a superior entity that is obsessed with him, proposes a deal, in exchange for her soul she would make him immortal, eternal.
The girl named Mor keeps his promise to him, but ...
You cannot die if you are already dead.
Our protagonist is nothing more than a miserable soul, a ghost.
A playful and troublesome spirit.
His story takes place in a fantasy world but I suppose this was a bit obvious if you read the labels, as I said, in this world there are both the demon king, as heroes and of course summoned heroes, but we do not follow any of them , our protagonist who has forgotten his own name and although he is not intrinsically evil, he is more attached to the side of an antagonist or an anti-hero.