From that it's possible everyone is a boss monster. If you look at screenshots the girl appears after monster is defeated. She was revealed before but not as a person who was turned form a monster.
Not everyone is a monster.
The player characters aren't, for starters. And neither are the bachelor and bachelorette officially revealed on the game's site (the manservant and the lutenist; I forget their names).
There'll probably just end up being two of each.