Instructions
Play through the game normally until you reach Chapter 6. At the end of the level, you'll be confronted by the mysterious Mr. L and his robot.
Defeat Mr. L by throwing bombs at his robot's feet. Don't worry about making other types of attacks, as the robot's feet are its weak points. In under ten attacks, he'll be finished.
Flee from the scene before everything explodes. Afterwords, you'll be treated to a long cut-scene, at the end of which the unmasked Mr. L joins your party.