Korrina is the leader of the Shalour City Gym. She uses Fighting-type pokémon and awards the Rumble Badge.

Gym Leader Korrina

Korrina has a level 29 Mienfoo, a level 28 Machoke (it knows the Rock attack Rock Tomb), and a level 32 Hawlucha.

When you win, you receive the Rumble Badge, which makes traded pokémon of up to level 50 obey you, and it also allows you to use Surf outside of battle.

Korrina also gives you TM98 Power-Up Punch.

Korrina says that to get the Mega Ring, you have to go to the top of the Tower of Mastery. She'll wait for you at the top, and when you get there she wants to battle you with her Lucario, using Mega Evolution.

To exit the gym, skate down where Korrina went. There is a rail down there that will take you to the exit of the gym. When you're ready, go back into the Tower of Mastery.