Unlockables
So, you wanna know how to unlock everything in Evil Zone? Most of them aren't too difficult, don't worry:

The Gallery - To unlock the Gallery (and the Extra options in general), complete Story Mode with any character.
Encyclopedia Entries - To acquire the Encylopedia for any character, finish Story Mode with that character; this will also unlock their third costume.
Ihadulca - To unlock Ihadulca and her stage, beat Story Mode three times with any three characters.
Narrator - To unlock the different Narrator voices, beat 1P Battle Mode.
Voice Collection - To unlock the collection of voice clips for a character, you have to hear as much of their dialogue as possible; the way to do this is to go through 1P Battle mode a few times with them, trying to finish off your opponents in different ways (with a super, with a throw, when near-death, etc) in order to get their different win quotes.
Intro Poses - Select a character repeatedly in VS (or, IIRC, 1P Battle). What they do and say when you select them will occasionally change; when all eight poses have been seen, they will be unlocked to select with the circle button.
Congratulations - This is the fun one. All you have to do is unlock every single one of the above for every character (the Intro Poses and Voice Collections will take the longest). Keep at it; it's worth it!

Oooooooorrrrr....

You could just use this Gameshark Code and unlock everything:
800A83DE FFFF
800A83E0 FFFF
800A83E2 FFFF
800A83E4 FFFF

But where's the fun in that?


Super Moves and Death Moves
In the game, when you have stocked a jewel (by holding down the attack button until your life bar fills up), you can use it to deliver powerful attacks that can turn the tide in battle.

Most of you probably actually already know about the Death Moves, which are used with Back-Back-Attack; it sends out a projectile that, if it is not blocked or dodged, will deliver your character's ultra attack (and an even more ultra attack if the damage will kill your opponent). However, what isn't pointed out in the instructions or many FAQs is that jewels can also be used to supercharge one or occasionally two of your character's ordinary moves by doing the usual motion for the move and holding down the attack button (instead of tapping it once or twice). You'll know you've succeeded if the character charges up their energy for a moment before striking with the enhanced attack.


Keiya Is Da Man
Keiya has a unique ability that no other character seems to share, namely the ability to instantly charge a jewel. Simply use Back+Attack+Attack (the red pentacle attack). If it successfully hits, Keiya will receive a jewel.