Clear the game and save your progress. Load that file on your next session and talk to the old man to unlock EX mode.

Clear the game and save your progress. Load that file on your next session and talk to the costume maker woman. You will need the item that allows you to exit the maze to do her mission. When you're ready, talk to the old man to start EX mode. There will now be different items you can find in the EX mode levels to unlock a new costume when you bring those items back to the costumer maker.

Each character has three different costumes that can be found. To get the final costume, defeat the Boss on EX mode level 100.