I still don't understand these spirits of "retired" players, maybe I missed something.
From what I understood, when a player "dies" in a dungeon they get expelled from it and lose all their player powers and wings/status, going back to being just a normal person. The exception for this are high level players, like the waifu, who get the chance to be reborn as Zelda's Navi when they die.
But then what's the deal with those spirits in the underground dungeons? How did they even get there when MC is apparently the only one who can access them? Are they players who got rekt in a sky dungeon, got spit out of it, fell from the sky, hit the ground so hard they ended up in a underground dungeon, then got killed for real there and transformed into vengeful spirits? If that's the case then this is a really shitty system as they would probably have spent the rest of eternity stuck in those dungeons if the protagonist never became a player.