Boss battle:
Defeat the last Boss to enable the "B" selection on the options screen. Choose this option to begin a battle with all six Bosses in order, with one life, and no chance to heal.
Graphics test:
Complete 100% of the game to enable the "?" selection on the options screen. Choose this option to view all the introduction to the game and all worlds and the various endings.
Regain health:
Summon your assistant, then assimilate him again to regain a small amount of energy.