Heaven has a single job: the regulation of the flow of souls into and out of this world.
If a soul does not flow from life to afterlife, from afterlife to rebirth, it stagnates and becomes foul. In pain from this neglect and fear, it grows monstrous and manifests as a Shell.
Long ago, Heaven would send its bureaucrats down to earth to sweep up these Shells and issue them back into the cycle of reincarnation.
But that age is over. Heaven is rife with petty internal struggles. And Shells have grown numerous.
You are an average person. Perhaps you are sensitive to spiritual energy, or perhaps your inquisitive nature has brought you in contact with the secret nature of the world.
Either way, you have learned to see Shells, and the soul inside you has reacted to them by manifesting physically.
As a weapon.
If Heaven won't do their job, then it's up to you.
REAPpEaR is a 26 page TTRPG about about regular people returning spirits to the cycle of reincarnation because heaven won't.
It is tonally influenced by Bleach, although I haven't actually seen much Bleach and so it's probably more accurate to say it's influenced by what I think Bleach might be.