In each zone, for every attraction (besides the Bomber Houses), complete each twice, so you get both the card AND the medal. Once you complete all the attractions for a zone, a new option appears in the Information Centers: SPECIAL. Click that, and pay 10 medals to unlock characters.

Code - Effect

Ao-bon (Blue Bomber) - Pay 10 Medals in Spade Zone
Gold Bomber - Pay 10 Medals in Diamond Zone
Kuro-bon (Black Bomber) - Pay 10 Medals in the Castle
Midori-bon (Green Bomber) - Pay 10 Medals in Club Zone
Pretty Bomber - Pay 10 Medals in Heart Zone
Shiro-bon (white Bomber) - Unlocked by Default