When you are saving the people at the Kouki foundation, there is a chest in the 
hotel. When you get it, it will say "Swimsuit". You can equip it to Shion or MOMO. 
This allows their TP reward to increase by 25%. Also, when they are fighting they 
will be in a swimsuit. If you saw the intermission sequence where they are at the 
foundation's beach, you will see MOMO and Shion in a bathing suit. That is how 
they will appear as when they fight with the swimsuit equipped. Also, the same 
item can be won by completing Mission 4 of the Driller Game. A third and final 
swimsuit is to be found by defeating Great Joe. Note: Jr. and Chaos can also wear 
the swimsuit, and can be seen in swimming trunks.