To unlock the characters below, fulfill the corresponding requirement. 

Asagi           - Clear Extra Map #4 
Axel            - Clear Extra Map #4 
Etna            - Clear Extra Map #7 
Flonne          - Clear Extra Map #7 
Laharl          - Clear Extra Map #7 
Marona          - Clear Extra Map #5 
Master Big Star - Clear Extra Map #1 
Prism Red       - Complete Extra Map #3 
Salvatore       - Complete Extra Map #2