After you finish the game, when the word "END" appears on the screen, press at the same time "A, B, Select, Start, Up, Left" on Controller 1, plus "A, B, Up, Right" on Controller 2. You'll get an extended ending with character profiles and some data regarding the enemies you've faced.
In a town, stand in front of the building where the save/warp Totems are and enter the building while holding Up + Right + B + A on controller 2. If done right, you'll get a sound test menu instead of the save/warp menu.
Avoid random battles
In any dungeon where you can use the Autopilot feature, set an autopilot checkpoint, walk a little, and then use the autopilot again to move back to the checkpoint. Before your character gets there, though, cancel his movement with B, and you won't engage random battles anymore as long as you remain in the same dungeon.
When you get any kind of Jewel from a chest, if you trade it on Rag's Store right away (before opening another chest) and then go back to that same chest, it will be closed again, and will contain another Jewel. Ginza B2 is the best place to exploit this glitch, as there's a shop near a chest with an Emerald.
Unlock The True Last Boss and Endings
To unlock the conditions to fight the true last boss and get the different endings, first defeat Bael in Bael's Castle and he'll turn into a frog. Take the frog and let him merge with Beelzebub in Beelzebub's Maze. They'll become Baal. Let Baal merge with Lucifer in Lucifer's Temple. If you fail to do all that, then the game will end prematurely when you beat Satan in Suzuki's Company and you'll get a bad ending. Now, if you followed the complete process, then you'll proceed to the real last boss (God). When you reach God, you'll have the option of fighting him for the 'good' ending or joining him for the 'bad' ending.
Increase MAX HP
If you use a Muscle Drink (health recovery item sold on shops) when your HP is full, you'll get a Max HP boost instead.
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