Play as Nine Tails Naruto - At the character selection screen, press X(2), Y(2), X, Y, X, Y, X(2) to unlock Nine Tails Naruto.

Bonus arenas - Successfully complete the indicated task to unlock the corresponding arena:

Coliseum Rooftop: Successfully complete Solo or Tag Tournament mode two times.
Endless Planes: Successfully complete Solo or Tag Tournament mode three times.
Hokage's Arena: Successfully complete Tag Tournament mode.
Infirmary Roof: Successfully complete Solo or Tag Tournament mode four times.
Madara Uchiha Statue: Successfully complete Solo or Tag Tournament mode five times.
Temple Courtyard: Successfully complete Solo Tournament mode.
The Valley Of The End: Successfully complete Solo or Tag Tournament six times.

Bonus characters - Successfully complete the indicated task in Story mode to unlock the corresponding character:

ANBU Itachi: Successfully complete Sasuke's second labyrinth.
Gaara: Defeat Gaara in Rock Lee's flashback.
Green Suit Naruto: Find the flowers mission for Guy.
Itachi Uchiha: Defeat Itachi.
Jirobo: Win the fight against Jirobo.
Kabuto: Defeat Kabuto.
Kidomaru: Defeat Kidomaru.
Kimimaro: Defeat Kimimaro with Naruto.
Kisame: Survive Kisame's attacks.
New Sasuke: Win the fight against Sasuke.
Nine-Tailed Naruto: Transform into Fox Naruto.
Rock Lee: Defeat Kimimaro with Rock Lee.
Sakon: Defeat The Sound Four with Sasuke.
Tayuya: Defeat Sakon.
Temari: Defeat Tayuya.
Tsunade: Make Tsunade Hokage.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

King Of The Iron Fist (5 points): Win a fight using a killer combo.
Solo Champion (25 points): Win a tournament in solo mode.
Team Champion (25 points): win a tournament in tag fighting.
Untouchable (20 points): Win a fight without losing any health.
Naruto's Creed (25 points): Complete all of the bounty hunter missions.
Card Collector (40 points): Find all of the ninja cards.
Delivery Boy (10 points): Complete all delivery missions with Shikamaru.
World's Fastest Ninja (15 points): Win all of the gold medals in the Tree Action Sequences.
Fisherman (15 points): Win a gold medal in any fishing game.
Gifted Eye (15 points): Complete all hide and seek missions with Neji.
Gold Collector (35 points): Collect 50% of the gold coins
Gold Digger (50 points): Collect all of the gold coins.
Legendary Fisherman (25 points): Win all of the fishing gold medals.
My Taijutsu Coach (20 points): Learn all of the combos in the Dojo.
Naruto Raving Ninja (25 points): Win all of the gold medals in the Tanzaku festival games.
Pervy Nation (20 points): Make all of the Lovelorn Villagers happy by performing the sexy jutsu.
The Last Chip Is The Best (20 points): Complete all gathering missions with Choji.
Top Dog (20 points): Complete all races with Kiba.
Way Of The Tree (20 points): Win a gold medal in a Tree Action Sequence.
Academy Graduate (15 points): Reach the Graduate rank online
Chunin (20 points): Reach the Chunin rank online
Don't Be Shy (5 points): Play a match online
Genin (15 points): Reach the Genin rank online
Hokage (0 points): Reach Hokage rank online
Jonin (30 points): Reach the Jonin rank online.
Save The Trees (15 points): Win an online match without performing a substitution.

Additionally, there are 20 secret achievements:

Tsukuyomi: ANBU Itachi unlocked (30 points): Complete Sasuke's Labyrinth.
The Last Stand (30 points): Keep Orochimaru from destroying the Leaf Village.
Green beast: Rock Lee unlocked (20 points): Defeat Kimimaro as Rock Lee.
Red Sand: Gaara unlocked (20 points): Defeat Gaara.
Broken Bones: Kimimaro unlocked (20 points): Complete the Naruto vs. Kimimaro fight.
Special someone: Temari unlocked (20 points): Complete the fight against Tayuya.
Guy's Fan: Green Suit unlocked (25 points): Complete Guy Sensei's quest.
Pervy Sage (30 points): Arrive at the Hot Springs Resort with Jiraiya.
Twisted music: Tayuya unlocked (20 points): Complete the fight against Sakon.
Deadly web: Kidomaru unlocked (20 points): Defeat Kidomaru.
Rock and Roll: Jirobo unlocked (20 points): Complete the fight against Jirobo.
New Hokage: Tsunade unlocked (20 points): Tsunade is the new Hokage.
Mad Scientist: Kabuto unlocked (20 points): Defeat Kabuto.
Brothers: Itachi unlocked (20 points): Complete the Itachi fight.
Lone Shark: Kisame unlocked (20 points): Survive the Kisame fight.
Hell twins: Sakon unlocked (20 points): Defeat the Sound Four as Sasuke.
Nine tails: Fox Naruto unlocked (60 points): Transform into Fox Naruto.
Broken Bond: New Sasuke unlocked (35 points): Complete the fight against Sasuke.
Path of darkness (10 points): Visit Sasuke secret room.
Seed of evil (10 points): Visit Orochimaru secret lair.