I found a Fire Keeper Soul in Undead Parish. Later, I went back to Firelink Bonfire, and a NPC offered me to upgrade my Estus Flask in exchange of the Fire Keeper Soul. I didn't pay attention, and accepted.
Now, I can see a +1 sign near my Estus Flask and it seems to give back somewhat more health. So do the Fire Keeper Soul item have other uses? Are they better or worse than upgrading the Estus Flask? How many time can I upgrade the flask, and how much do each upgrade change the HP regeneration?