Did you trigger the Maneater Mildred invasion? It's right in front of the first bonfire in blight town while your human. You can summon her for help with Quelaag.
After you ring both bells of awakening, go to the gate in front of the undead parish. From the bonfire in the blacksmiths building, head straight out. You'll find Siegmeyer the onion knight there. So remember that.