There are 2 ways to unlock Mr. 2 Bon Clay. The first way is after you unlock Mihawk, finish Story Mode with every available character on every difficulty setting (Easy, Normal, Hard, and Very Hard. Make sure you have at least 40 hours of accumulated gameplay at this time. Finally, beat Story Mode with Sanji on the Very Hard difficulty setting. Bon Clay appears as the final boss, and is selectable in Story Mode if you defeat him with you Secret Attack. Finish Story Mode with Bon Clay, and he becomes selectable in all other modes. The second way is easier. Beat Story Mode with Sanji on any difficulty level. During the ending credits, enter this code:
Hold L1 and press: X, X, Circle, X, Left, X, Circle, Square, X, Circle, Triangle, Circle, X, R1, R1, Left.