Sadly, that was Laurentius, the pyromancer you save from being eaten by the butchers in the Depths, and who teaches you at Firelink. After you meet Quelaana and have her ascend your pyromancy flame, the next time you interact with him he will remark on your "spectacular pyromancy", and ask you where you got it. If you tell him, he leaves Firelink in search of Quelaana, as learning from the mother of pyromancy and becoming a master of the art is his dream. But somewhere along the line he goes mad in the swamps. It's impossible to know if it's because his pyromancy isn't strong enough to see Quelaana, and loses his mind, or if it's because he DOES find her, and either the deepest pyromancies are too much for him or he hollows after realizing he has nothing left to learn. Either way, if you don't want to have to kill the poor guy the next time you go to Blighttown, you do not tell him where you got that sweet sweet fire. He will respect that decision and trust you, remaining in Firelink.