Giant Robo: The Animation - Chikyuu ga Seishisuru Hi

Finish Battle Mode once. Genya will be available as a pilot on VS Mode.

To unlock other playable robots, you have to collect a certain ammount of Robo Parts. They're those blue items that are scattered around the floor when you attack the enemy robot. You can collect them in either Battle or VS Mode. Just be careful that you need to win the fight so the game saves all the Parts you collected, and the enemy can also steal them off you, if your pilot is attacked directly.