How do you unlock grimmjow in bleach the 3rd phantom

Grimmjow is hard to unlock, but unlockable. After completing the game you get the choice to try the Bleach Tower which is a series of battles, 30 in total, one after the other. You need to defeat a level90 Grimmjow on the final floor to unlock him. This also bags you Aizen after defecting. Hope that helps. Need any more info on secret characters check out the game spot forums for help.