Completion bonuses:
Successfully complete the game to unlock options for stage select, sound test, and event test. Successfully complete the game two times and wait after the credits complete for a sequence that unlocks Sharaku Hosuke as a selectable character.

Gunstar Heroes reference:
Listen to BGM 30 once you have unlocked the sound test option. It is the same music from the final stage of Gunstar Heroes. Both of these games were developed by Treasure Games and produced by Sega.