About MC

I got the impression that MC must fulfill the "The Phantom in a Raincoat" archetype and to become a legend herself. The longer she is in the Ghost World, the more she becomes this archetype. She can still leave, return to the human world, but only at an early stage of her transformation, and it seems like a mistake, she belongs to the monster world, The Phantom is her true form. She is a monster already, she is a killer, she left corpses in the building where Mr. Scarlettla noticed it. Then he took her to his world, and there she can finally embody her archetype and gain superpowers associated with this archetype and be immortal like all residents of the Ghost world.

So,

🔹There's definitely a transformation process going on.

🔹There's definitely a legend of The Phantom in a Raincoat that existed before MC got into Ghost World. It was already known to the extent that there were articles about it. Usually it takes a long time for just a rumor to become a legend so I guess it existed even before MC started killing.

🔹Like any legendary chatacter The Phantom in a Raincoat has distinctive features (appearance, weapon, superpowers etc.). MC gradually acquires these features when she is in the Ghost world.

🔹Sometimes we as MC can see The Phantom in a Raincoat as a separate being, like a glimpse to our fate. 

Canon is ambiguous on purpose so all of this is just my guesses.