Alternate costumes - At the character selection screen, highlight a fighter and press Y.
Attack Power 4 - Finish in first place in Time Attack mode to unlock Attack Power 4.
Difficulty 4 - Finish in first place in Survival mode to unlock Difficulty 4.
Easy wins with Naruto Kyuubi form - To win easily press Forward, B(5). Before your opponent gets back up, use it again. If you get hit, press R and use Substitution Jutsu. You will probably have lots of Chakra. Note: You must have a green Chakra level to perform Substitution Jutsu.
More shuriken/projectiles - Hold A to charge up shurikens/projectiles to throw more of them.
Play as Kyuubi Naruto - Unlock Rock Lee, then successfully complete Single Player mode two times with all characters. Then, highlight Nartuto at the character selection screen and press X (for the orange suit Kyuubi) or Z (for the red suit Kyuubi).
Play as Rock Lee - Play the game in Story mode. Use no continues and defeat Zaibuzan. Defeat Rock Lee when he appears to unlock him.
Play as Sharigan Kakashi - Unlock Kyuubi Naruto, then successfully complete Single Player mode with him. Then, successfully complete Single Player mode with Kakashi. Highlight Kakashi at the character selection screen and press X or Z.
Play as Unmasked Haku - Successfully complete Arcade mode with Zabusa on Difficulty 3. Then, highlight Haku at the character selection screen and press X or Z. When Haku is hit in the face, or is hit and takes a lot of damage, Haku's Mask will fly off his face.
Projectile combo with Sakura and Haku - With Sakura, after pressing A for shuriken, press A again for another, then A again for a finsher to the combo. Haku can do sort of the same thing, however after pressing A initially you can only press A once more.
Sasuke's combo - One of Sasuke's best combos is B(4), A. The Fire Jutsu causes a lot of damage and if timed correctly, can kill the opponent.
Sound test - Successfully complete Single Player mode with all default characters to unlock the "Sounds Test" option in the Omake menu.
Stronger special with Kakashi - When performing Kakashi's special, hold Y for it to charge and him to attack automatically. The attack will go farther and do more damage.
Use enemy's super with Sharingan Kakashi - When your opponent uses their super, use your counter. If done correctly, Kakashi will do his own version of your opponent's super.