I forgot to ask about classes.

Everybody knows that the inhabitants of the underworld are transformations of dead human souls, right? However, they will not remember anything from the human world, but their personality and such will be retained.

The human world and the underworld cannot interact except, of course, through the Grim Reaper, whose job is to guide dead humans. Since humans are rich with spirits right after death, the Reaper has to reap any extra spirits between their death and them entering the underworld.