Instructions
Play the game in "Arcade Mode." When you go to the game's main menu after loading it, flip through the options until you get to "Arcade Mode" and press "A" on your Wiimote.
Select any difficulty for this mode; it has no bearing on whether you unlock Ichimaru. When prompted to pick a character, pick either Matsumoto Rangiku or Kira Izuru.
Beat "Arcade Mode" with the character you selected. You have to fight eight random "Bleach" characters to beat this mode.
Return to the main menu after you have beaten "Arcade Mode" and start the process over again. When you are prompted to pick a character, choose the one that you didn't pick the first time. For example, if you chose Rangiku the first time, pick Izuru this time. Beat "Arcade Mode" again to unlock Ichimaru.