Play As Hawk-eyes Mihawk:

Unlock Red-haired Shanks, then successfully complete Story mode with Roronoa Zolo on the very hard difficulty setting. Use a secret attack to defeat Hawk-eyes Mihawk to unlock him. In the Japanese version, successfully complete Event Battle with Zoro on the very hard difficulty setting after Shanks has been unlocked unlocked. Defeat Mihawk with a level 2 special after getting hit with Mihawk's level 2 special to unlock him.