How do you get the item to unlock the other characters?

[PowerElite]

Topic

[Legoshi]

i think you gotta beat the boss , its an abyss stone so im assuming its that, i havent got one yet

[Kwazymofo]

Yep, Legoshi is spot on. The item you are after is called an abyss stone. I got one when I defeated the first boss (the werewolf looking one that is the cheapest to attempt).

Not sure if the boss itself matters, but the wolf one is the only one I've beaten so far.

Also just fyi. After I got an abyss stone, the option to unlock a character disappeared entirely. I had to quit to the main menu and load back in to make it reappear.