Final Fantasy X-2/Trainer

During Chapter 2, visit Kimahri on Mt. Gagazet and speak to him twice. He will ask you a question, which you must pick the second option for. Then you have to wait until chapter 3.

During Chapter 3 you will have a mission that takes you to Mt. Gagazet. Speak with Kimahri again and he will hand you the Dress Sphere.

Abilities
When Yuna wears the Trainer Dress Sphere she summons her company, Kogoro, to attack for her.
 * Yuna