Apprentice (Hero): An alternate version of Robin,

Beast Boy (Hero): Teen Titan capable of shape-shifting into animals,
Blackfire (Villain): Starfire's sister; plays exactly like Starfire,
Blocker (Henchman): Robotic fighting machine,
Bumble Bee (Hero): Uses powerful twin lasers,
Bunnyguard (Henchman): Mumbo Jumbo's bodyguard,

Cinderblock (Villain): Made of concrete and stone, strongest character in the game,
Clown Beast Boy (Hero): An alternate version of Beast Boy,
Control Freak (Villain): Attacks with a variety of sci-fi gadgets,
Cyborg (Hero): Part-robot with enormous strength,
Cyclone (Hero): An alternate version of Cyborg,

Dr. Light (Villain): Uses the power of light to attack,

Fang (Villain): Plays the same as Gizmo in his mecha-spider,

Gizmo (Villain): Attacks with mechanical contraptions of his own design,

Hive Soldier (Henchman): Works for the Hive Academy, a school for supervillains,
Hotspot (Hero): Uses powers of flame to attack,

Jinx (Villain): Jinx's powers of bad luck are translated into powerful energy attacks,

Mad Mod (Villain): Wears an exoskeleton designed after the London Royal Guard,
Mammoth (Villain): Uses his great strength to defeat his enemies,
Mega Meaty Meat Beast Boy (Henchman): An alternate version of Beast Boy,
Mumbo Jumbo (Villain): A diabolical magician,

Plasmus (Villain): Blob monster army leader encountered during story mode,
Plasmus Kid (Henchman): A fighting blob monster,

Raven (Hero): Fights with ranged telekinesis attacks,
Red Raven (Hero): An alternate version of Raven,
Red X (Villain): Robin's alter-ego come to life,
Robin (Hero): Acrobatic melee fighter,

Slade (Villain): Arch-enemy to the Teen titans, clever fighter,
Slade Minion (Henchman): Generic foot soldier in the employ of Slade,
Speedy (Hero): Attacks with bow and arrow,
Starfire (Hero): Powerful ranged beam attacks,

Tempest (Henchman): Robotic fighting machine,
Terra (Hero): Geomancy powers let her attack with rock projectiles and landslides,
Trigon (Villain): Raven's demon father fights with flame attacks and powerful punches,

White Raven (Hero): An alternate version of Raven,
Wildebeest (Hero): Powerful man-beast.

Concept Art  -  Each Larry the Titan you find in a level, unlocks concept art in the gallery extras menu.

Multiplayer Mode Stages  -  Just play through Story Mode to unlock Multiplayer Mode Stages.
Multiplayer Mode Stages - Play through Story Mode

Multiplayer Mode Characters  -  All you have to do is just play through Story Mode and eventually unlock characters.
Multiplayer Mode Characters - Play through Story Mode

Unlockable Arenas

City Cathedral - Complete City Night Showdown Level
Larry's World - Complete Danger Yard Level
Mad Mansion - Complete 1st part of Jail of Horror Level
Mumbo's Big Top - Complete 2nd part of Magic Mayhem Level
The Hive - Complete Hive Academy Level
Raven's Mirror - Complete Thriller Driller Level
Sky Needle - Complete Titan Tower Level
Titan Tower - Complete Slade Ambush Level
Slade's Lair - Complete Final Clash Level