TO get all 5 secret characters you first have to beat the game. You
can only get one of the characters before you beat the game and that's
Ezel, but its almost impossible so you might as well wait.

Shara-Clear the A maidens Cry mission, then enter a town

Ritz- Clear the Mortal Snow mission

Ezel- Choose gossip in the card shop, then clear the Reconciliation
mission

Babus- Clear the Left Behind mission, then enter a town and clear the With
Babus mission

Cid-Clear all 300 numbered missions