Board Thread:General Discussion/@comment-74.104.206.83-20130730235710/@comment-23784953-20130922144141

That's what I was thinking actually. Throughout the game, you'll find fragments of data pads that you can stitch together to unlock a secret mission related to an older character.

Then if you complete said mission you'll unlock him/her.

For example, you can complete a data pad regarding Glynda's (possible) first battle with Crimson. If you finish it, then you can use Glynda on replay missions.