Each of the Diez Gentlemen can be acquired by starting a New Game+ while Mao is at at a certain level or above.

Unlockable	How to Unlock
Beyond X	Level 64
Bo Tie	Level 300
Breezy Satina	Level 700
Gorillian	Level 64
Goro	Level 400
Pauline	Level 600
Prinny Mask	Level 64
Se-to-oh	Level 500
Shyrone	Level 64