Talk:BlazBlue: Cross Tag Battle/@comment-27144409-20180620235550/@comment-10380701-20180621000448

Keystones are kinda explained at the midpoint of the Blazblue story mode. They just happen to be in the hands of the main character once the entire shebang starts.

Due to System witholding specific information and outright lying in certain story modes, everyone is after each other's Keystones since the general consensus is you and your franchise get to go home if you collect all of them.

The rest are spoilers if you're willing delve deeper into it.