Enter the following codes at the password screen

Abyss:
S, D, S, D, R, L, D

All-You-Can-Throw Shuriken:
R, S, R, L, R, S, M

Challenge Mode Endurance:
M, R, M, D, R, M, D

Fatal Blow:
L, R, S, R, D, R, D

Health:
D, S, R, D, M, R, M

Lose Shuriken
R, L, M, R, D, S, L

Mighty Turtle:
L, S, D, R, R, D, R

New Nexus Turtle Outfit for Donatello:
D, S, L, R, D, R, M

New Nexus Turtle Outfit for Leonardo:
L, M, R, M, D, R, D

New Nexus Turtle Outfit for Michelangelo
M, L, M, R, D, R, M

New Nexus Turtle Outfit for Raphael:
R, M, S, R, M, D, R

Nightmare:
S, L, S, D, R, D, L

Pizza Paradise:
M, R, L, M, R, M, R

Playmates added to Bonus Materials:
S, R, M, L, D, D, R

Poison:
D, R, S, L, L, S, R

Self Recovery:
D, R, M, S, R, L, R

Squeaking:
M, L, D, S, R, D, M

Super Defense Power
L, D, R, M, R, L, M

Super Offense Power (Attack x2):
S, D, L, S, R, L, L

Super-Tough:
R, D, S, R, M, R, L

Toddling:
S, S, S, M, R, D, D


Unlockable Hidden Characters
Complete the following tasks to unlock the corresponding characters

Unlock Casey Jones:
Beat the Open Brawl Tournament (Replaces Raphael)

Unlock Splinter:
Beat the game (Replaces Donatello)

Unlock Karai:
Defeat her in the Foot Fight Tournament (Replaces Michelangelo)

Unlock Slashuur:
Defeat him in the Battle Nexus Tournament (Replaces Leonardo)


The following is the alternate way to unlock the hidden characters.

Unlock Casey Jones:
Unlocks automatically after 5 days (Replaces Raphael)

Unlock Splinter:
Unlocks automatically after 10 days (Replaces Donatello)

Unlock Karai:
Unlocks automatically after 35 days (Replaces Michelangelo)

Unlock Slashuur:
Unlocks automatically after 45 days (Replaces Leonardo)