In this AU Grimmer is an ajin. Ajin are immortal and ppl typically find out they’re one when they’ve died. Ajin are generally treated terribly so people generally hide this fact if they can help it. Otherwise they can be subject to human trafficking (for their organs) or placed under government ‘care’.
In this particular AU, Grimmer is an ajin placed under government care who voluntarily undergoes medical testing and Dr. Tenma, brought in for his talents, is one of many doctors overlooking the process. However, he comes to find that what they’re doing is morally unacceptable (do no harm) and on top of that Grimmers consent is somewhat questionable, so he takes things into his own hands.
Another aspect of ajin, is that they have something sort of like ghosts attached to them that are black, human-like and ribbon-y in structure. These ghosts have superhuman strength and can be summoned at will. Normally they can’t be seen by ordinary people unless there’s a strong emotion directed at them.