>>2757222>spoilerfine, chuuni shit is more fun anyways.
Going off of lore dropped via Q&A, moments in stream, and Takamori...
Pre-idoldom, Mori was a reaper apprenticed to death-sensei who was capable of inter-dimensional and time travel (most likely via hell portals) she was athletic by the standards of hell denizens and even has multiple invisible limbs.
Her main passions were underworldian rap and art.
She also had to do scythe swinging lessons for newbie reapers since her main occupation didn't pay her enough, but through her working with them she's grown a soft spot for kids.
Personality wise, her occupation as reaper and her hellish life in hell she was quite cynical on top of being a near unstoppable workaholic business lady.
Pre-reaper, we don't really know how hell denizens are chosen for certain roles but based on the fact that reaper school includes general education on top of reaper education it can be deduced that they're called to do the job at a young age and depending on if they accept the mantle and some sort of test and/or if a reaper chooses them as an apprentice they either end up becoming apprentices, deny the mantle, or fail the test and/or choosening.
My money is on the fact that she barely succeeded in becoming a reaper and that death sensei choosing her as an apprentice was a surprise to her and everyone else.