If you played the game before and you dont see the full 7 characters (6 that are initially black)

You have to go to
C:\Users\ (USER NAME) \AppData\Local\SlimeMasters\Saved\SaveGames
and delete
DataFile.Sav

You will lose your progression but you will be allowed to use the character.
The structure of the character selection have been changed, this is why the bug occur.

It wont change anymore!