Not sure if there was a game update that threw things off or what, but as of June 2021 there are actually seven characters, but you only have to unlock the first six to get this achievement. Just keep playing the game, and in between dungeon runs you will usually get a cutscene that progresses the story of the Bergson family. It will introduce new family members along the way, and after one or two more between-run cutscenes with them they will eventually be unlocked on your character select screen.