Laurentius of the Great Swamp

Located in a room near two butchers in The Depths. After taking care of the first butcher, go to the stairs that lead to waist deep water. You should see a part of the wall crumbled down, which you can climb up. Laurentius is located in a room to the left. He is trapped inside a barrel, roll into the barrels to release him.

After you release him, you can return to Firelink Shrine, where he will give you a Pyromancy flame and offer to teach you several Pyromancy Spells.

Laurentius can also upgrade your Pyromancy flame in exchange for souls. This will increase the effectiveness of your fire spells.

If you upgrade a pyromancy glove to +15 with Quelaana (the item description will change), he will ask you where you found it. He will also ask this if you received the Great Chaos fireball pyromancy. If you tell him, he will shortly leave Firelink Shrine and can be found on the ground level across from the second bonfire in the Blightown swamp as an enemy hollow. He drops 1000 Souls upon his defeat.

However, you may simply answer "no" to his question and he will not leave the Firelink Shrine and will still sell you pyromancies.