Unlock Extra Characters
Daichi - Beat Daichi and win tournament mode.
DJ - Beat DJ and win tournament mode without using any continues.
King - Beat King and win tournament mode.

Hint: Best Parts
The best parts are:

Bit Chip: Dragoon V2
Attack Ring: Upper Claw Gamma
Weight Disk: Magine Weight Disk Gamma
Spin Gear: NEO Right Spin Gear MWV Gamma
Blade Base: Switch Metal Ball Base Gamma
Launcher: One of the Ultimate Launchers

Note: You must beat the tournament before some of these parts become available