Oritsuru » Final Fantasy X-2 » Dress Spheres » Dark Knight
A Dark Knight's attack strength is much like that of classes such as the Warrior. High attack and defense but low resistance to magic. Unlike a knight, the dark knight's abilities focus on causing various status effects to fiends such as Darkness and confuse. This could come in handy, however, many of the monsters in Spira are immune to these attacks. One awesome thing about a dark knight is that they can learn auto abilities such as confuseproof to protect them from status effects.
If you remember from Final Fantasy X, the status attacks are much like Wakka's, only causing diffrent status effects. You can get this dressphere in the Bevelle Labyrinth in Chapter 2.