An Easy Option to Unlock Itachi
Begin a game in "Heroes" mode. In this mode you'll unlock courses one by one and will have to choose a team of ninjas to use. Play through with any team until you fight and defeat Itachi to unlock his character.
An Easy Option to Unlock Kisame
Continue playing through the game in "Heroes" mode until you reach Kisame. Defeat Kisame with your team of ninjas to unlock his character.
A More Difficult Method to Unlock Itachi &Kisame
Start a geme in "Heroes" mode and select "Insane," the most difficult setting. Begin and complete the level called either "Atatsuki Returns" or "Atatsuki's Invasion." Completing this level on "Insane" difficulty will unlock both Itachi and Kisame.
Unlocking Itachi and Kisame in the Japanese Version
This method will only be effective if you have the Japanese version of this game. Begin a game in "Second Story" mode. Clear 30 floors of enemies, and after you have defeated all enemies, the characters of Itachi and Kisame will be unlocked.