>>3590251An executioner may not be a bad comparison. Like an old one (or even in some parts of the world today) where it was an actual profession. One with a fair bit of dignity and respect associated, but the actual nature of the work meant many kept away from those who did it. As such they ended up isolated except from those who were similar. Mori is like that crossed with a bounty hunter, since some souls may be harder to take than others and require more skill.
The idea that any Underworld resident can become a Reaper is a good one. I think Mori's backstory would involve her rise being somewhat of a mistake, where she was even shocked and unsure of herself. Her master saw qualities in her that he believed were fitting. Reapers may not rule the Underworld but they play a very important part.