Explodemon

You need to purchase all of the upgrades from the store, collect all of the Gold Speedchargers and unlock his abilities from progressing through the game.

Gold Speedchargers

There are four to be found throughout the game and the fifth is bought from the store for 5000 credits. Following the Collectibles Guide makes this a lot easier.

Store Upgrades

The first upgrade for each costs 300 credits and the second costs 800, requiring a total of 5,500 to purchase all of them.

Explosion Power
Time Before Explode
Armor
Boost Period
Charge Gain

Abilities

These are gained as you progress through the story and come free, so nothing to worry about here.