Get S Rank in all 50 missions in the extra battle review mode (unlocked after beating the game) and you'll unlock this mode that gives you infinite heat gauge during gameplay.
8/29/2006
Secret Boss Fight
Complete all the side quests and a secret boss will appear in front of the movie theater. Talk to him to initiate the fight.
9/22/2006
Easy Game Mode
To unlock easy mode, keep dying on a certain fight, die a couple of times and keep retrying(5-10) until they ask you "You want to select to easy game mode?"Also take note that when you select this easy mode you won't be return to normal mode anymore.Advantages: Number of enemies will decrease, they only inflict minimal damage and they attack only one at a time