Karla is a new NPC in Dark Souls III, but she can be quite easy to miss. Before you can free her in the Irithyll Dungeon, first you need the Jailer’s Key Ring located in the Profaned Capital. From the Profaned Capital bonfire, take the ladder that first led you up to it, and then head out from the entrance on the ground floor. When you reach the toxic swamp with the spider ladies, cross the swamp and head upwards. Ignore the enemy on the roof, and head inside until you enter the dungeon area with the giant. The Key Ring will be on the ground.
After getting the Key Ring, go back to the first bonfire in Irithyll Dungeon. Make your way to the ground floor where you’ll find several of the brand ladies patrolling the area in a circle. Karla is locked up in the cell next to the one with the red-eyed crucified enemy. Talk to her to get her to teleport to Firelink Shrine.
Karla will accept the Izalith and Grave Warden Pyromancy Tomes. If asked repeatedly, she will also accept the Londor and Deep Braille Divine Tomes, so you won’t have to give them to Irina to learn those spells.
Be sure to check out our Dark Souls III wiki for more tips and tricks.