@Micro
Well so far we only know that she can only bee seen by the MC.
That doesn't mean she can't be a revenant.
Revenants are basically just a undead with a body, but have latelly been depicted as ghosts with corporeal forms too.
They tend however to hate the living or have a mission they follow with blind zeal so it's a bit out of character for them to make deals like she does.
But that could just be a personal trait of hers as revenants usually keep some of there personality even after death.
Also just apply magic to fix all your plot hole needs, if the author wants the MC to mary a ghost, then he can make it so.
TL
R there is nothing that says she doesn't have the ability to interact with the living world, just that only he can see her..... also just apply magic to fix all your plot hole needs.
ghost waifu is best waifu.