Character, stages, and clips...
	Characters...
	Various Trophies...

Go to options, and click on the button labeled CHEATS.

Effect	Code
Unlock alternate costumes for every Character	MONSTER
Unlocks 5 of the Battle Arenas	MISSY
Unlocks 8 Characters	OSCAR
Unlocks all Levels and Characters in Store Mode	SUMO
Unlocks All Tv Clips in Store	RUFUS

Most character unlocking is done in Story Mode, but a few have to be bought in the Store. Difficulty does not matter when playing through Story Mode.

Character	How to unlock
Aku	Buy in Store for 1500 points.
Blossom	Beat Kids Next Door Theme in Story Mode.
Bubbles	Beat Kids Next Door Theme in Story Mode.
Captain K'Nuckles	Meet him in Stage 1 of FlapJack Theme in Story Mode.
Captain Planet	Beat Samurai Jack Theme in Story Mode.
Father	Beat Story Mode.
Grim	Beat Dexter's Lab Theme in Story Mode.
Him	Buy in Store for 1200 points.
Hoss Delgado	Buy in Store for 1200 points.
Johnny Bravo	Buy in Store for 1500 points.
Kevin 11	Buy in Store for 1200 points.
Mojo Jojo	Beat Story Mode.
Monkey	Save him in Stage 1 of Dexter's Lab Theme in Story Mode.
Samurai Jack	Beat him in Stage 1 of Samurai Jack Theme in Story Mode.
The Scotsman	Buy in Store for 1200 points.
Toiletnator	Buy in Store for 1200 points.
Vilgax	Beat Story Mode.
Young Ben	Buy in Store for 1500 points.

Complete the following tasks to unlock PlayStation 3 trophy rewards.

Trophy	How to unlock
"Yes!" (Bronze)	Activate Ben's synergy assist.
A great Big Hug (Bronze)	Activate Bubbles' synergy assist.
Adventure! (Bronze)	Win the first Flapjack Pier level in story mode with Flapjack and K'nuckles in under three minutes.
AH-ha-ha! AH-ha-HA ha-ha! (Bronze)	Activate Mandark's assist while fighting against Dexter.
Ben 10 Fan (Bronze)	Finish the Ben 10 theme in story mode.
Big Money! (Silver)	Collect 1000 points from a single level.
Billy & Mandy Fan (Bronze)	Finish the Grim Adventure's theme in story mode.
Cartoon Collector (Gold)	Unlock all characters.
Chicken Ball Z (Bronze)	Score four knockouts after using Billy and Mandy's PTE.
Chowder Fan (Bronze)	Finish the Chowder theme in story mode.
Costume Collector (Gold)	Unlock all costumes.
Couch Potato (Silver)	Beat Story Mode.
Dexo Robo 2000 (Bronze)	Score two knockouts using Dexter's PTE.
Dexter's Lab Fan (Bronze)	Finish the Dexter's Lab theme in story mode.
Dial M for Monkey (Bronze)	Win ten fights as Monkey in his alternate costume.
Disco Fever (Bronze)	Score two consecutive knockouts using Captain Planet's synergy assist.
Do the Monkey (Bronze)	Activate Johnny's synergy assist.
Father is Number One (Bronze)	Win five fights as Father with Numbuh One as an opponent.
Flame Broiled (Bronze)	Score a knockout using Father's synergy assis.
Flapjack Fan (Bronze)	Finish the Flapjack theme in story mode.
Flushed (Bronze)	Score two simultaneous knockouts using Toiletnator's PTE.
Foster's Fan (Bronze)	Finished the Foster's Home theme in story mode.
Freedom!!! (Bronze)	Win the Toadblatt's level in story mode with Grim in under two minutes.
Frenemies (Bronze)	Defeat Kevin boss using Ben without dying.
Go Mac Go (Bronze)	Win ten fights as Mac and Bloo in their alternate costumes.
Him Diddle Riddle (Bronze)	Win ten fights as Him against a Powerpuff girl.
I am not your Boyfriend (Bronze)	Activate Panini's assist while fighting against Chowder.
It's Hero Time! (Bronze)	Win the Bellwood fight in story mode with Ben or Young Ben in under a minute.
It's over 9000! (Silver)	Collect 9001 points.
KND Fan (Bronze)	Finish the KND theme in story mode.
Like a Hoss (Bronze)	Win the Endsville Cemetery level in story mode with Hoss Delgado without dying.
Master of Darkness (Bronze)	Win five fights as Aku with Jack as an opponent.
Meet My Beautiful Wife (Bronze)	Activate Scotsman's PTE and Peppermint Larry's Assist in the same fight.
Ninjas Beat Pirates (Bronze)	Win a fight as Jack in his alternate costume with Flapjack and K'nuckles as opponents.
No Big Deal (Silver)	Beat Story Mode on the Hard difficulty setting.
Not Powerpuff, Rowdyruff! (Bronze)	Win a fight with each Powerpuff in alternate costume against a Powerpuff in normal outfit.
P90 Him (Bronze)	Win nine fights as Him in his alternate costume.
Planet of the Ape (Bronze)	Win ten fights as Mojo Jojo against a Powerpuff girl.
PPG Fan (Bronze)	Finish the Powerpuff Girls theme in story mode.
Professional Hero (Silver)	Defeat the Final boss without dying.
Punchtime Explosion Pro (Platinum)	Earn all trophies.
S.E.C.T.O.R. V. (Bronze)	Complete Kids Next Door Platforming in under five minutes.
Samurai Jack Fan (Bronze)	Finish the Samurai Jack theme in story mode.
Samurai's Journey (Bronze)	Defeat Aku boss using Jack without dying.
The power is yours! (Bronze)	Score three simultaneous knockouts using Captain Planet's PTE.
Ugly Finish (Bronze)	Score four consecutive knockouts using Kevin's PTE.
Unreal Estate Collector (Gold)	Unlock all arenas.
Vengeance of Vilgax (Bronze)	Win five fights as Vilgax against Ben and Young Ben.
Viking God of Rock (Bronze)	Score two consecutive knockouts with Aku's synergy assist.
Way of the Samurai (Bronze)	Score five knockouts after activating Jack's PTE.
What Does This Button Do? (Bronze)	Activate DeeDee's assist while fighting against Dexter.