Fire keeper's soul importance?

[Qwertion]

I accidently consumed it. I thought it should upgrade my flask that way. Now I know I was wrong. How costly was my mistake? Is it viable to restart the whole game just to correct it at this point?

[Tusk- Mann of Peace]

It's probably been said at this point, but Fire Keeper Souls are used in two ways. One is by the player, and the other is by giving them to an NPC. When consumed by the player, it gives five humanity, and I don't know for sure, but it might heal you as well, but I can't say for sure.

The short version is this: use the Firekeeper Soul in your inventory and you get 5 Humanity. Speak to the people by the bonfires at Firelink Shrine, the area after Queelag's fight, and the bonfire in Anor Londo that's by the elevator, and you can make your Estus Flask stronger. You can reinforce your Estus Flask a total of 7 times before you can't do it anymore. Once you've done it seven times, you can just use Firekeeper Souls to get 5 humanity. You usually do this in New Game + though. And don't worry if you've accidentally used a Fire Keeper Soul, there's more of them. Having an Estus Flask +6 is just as good as one at +7.

(This next bit is just stuff about the Firekeepers and where they are. If you already know this then you don't have to read this, it's just me rambling.)
When given to an NPC Firekeeper, such as the mute woman behind the bars at Firelink Shrine, it reinforces the Estus Flask. There are three Firekeepers in the game. The first is the woman I just mentioned. The second is Queelag's Sister, who's also in charge of her respective covenant, and is who the Old Witch Ring is used for, as it lets you talk to her. The third and final Firekeeper is the woman in the Brass Armor set at Anor Londo, who leans on the wall by the Bonfire near the elevator and the stairway that leads to that bat thing that takes you back and forth from Anor Londo and the tower you fight the axe-wielding giant at.

[zero hope >:(]

@Tuskor, why don't you tell OP the whole your biography too while he listens to you?