No cheat code unlocks all characters instantly in the PS2 version. The PS2 port (by n-Space) differs from PS3/Xbox 360/PC and has no documented cheat menu.
24 base + 4 simulator = 28 total (vs. 30+ on HD consoles). Missing: Green Goblin, Venom (HD exclusive).
Action Replay Max / CodeBreaker codes exist on GameHacking.org but are unstable — often freeze saves. Use at own risk; backup save first.
Recommendation: Play through — the story is the game's strength.