Firekeepers in Dark Souls - what is their place in the Dark Souls Universe?

So I have been watching some playthroughs lately of the Souls Games, and the one thing that has stood out to me was the way that the Firekeepers are treated throughout the different games.

For example, in DS1, there are 3 "active" Firekeepers:

• Anastacia of Astora
• Daughter of Chaos (Queelag's Sister)
• Darkmoon Knightess

There is also several Firekeeper souls found throughout the game, like the one in the Undead Parish, found on an Altar that Rhea of Thorolund eventually visits; whom is also attemtpting to become a Firekeeper.

The Firekeepers are responsible for their associated Fires - Firelink Shrine, Quelaag's Domain, and Anor Londo; as well as assisting with upgrading your Estus Flask, for which you use Firekeeper Souls to reinforce.

In DS2, the "Firekeeper" Role is significantly changed. Not only does the player need to reinforce the Estus flask with Shards, instead of using the Rite of Kindling (as they do in DS1), but to reinforce the Flask uses "Sublime Bone Dust" instead of the Firekeeper Souls, which can only be done in the Fire at Majula.

Additionally, the actual "Firekeepers" do not attend any fires at all, (not the Emerald Herald, as she does not "attend" to the fire at Majula, as is the case for the Firekeeprs in DS1) and killing any of them does not affect any fires in the game, and only rewards the player with Human Effigys/Aged Feather.

In DS3, everything revolves around the Firekeeper. She attends to the "oldest" Fire - in Firelink Shrine, and the endings are determined by interacting with her through the items "Eyes of a Firekeeper", and the "Firekeeper Soul". These items are key items, and reinforcing the Estus Flask happens the same way as it does in DS2.

And again like in DS2, killing her does not extinguish the Fire in Firelink Shrine.

So all in all, in DS1, Firekeepers are established to be important to the Age of Fire. While they are Undead; they are specifically linked to fires throughout Lordran, and all appear to be in favor of Linking the Fire.

In DS2, the Firekeepers have all but abandoned their role and been forgotten, and an unrelated Herald exists in their stead, simply acting as a tool for others - unrelated to the Firekeepers and the Fire itself, but again, seeking to Link the Fire.

Then in DS3, the only remaining Firekeeper serves only the Fire, and the one linked to it. She is all important and central to the plot of the game, and held in the highest regard by any who are involved in the main plot of the game. Her interactions involving the "End of fire" plotline also reflect Anastacia's responses to reviving her after she is killed by Lautrec, and then revived, returning her speech (emphasis mine):

Anastacia: Forgive me… I am impure, my tongue never intended for restoration. Please, if you have any heart… Leave me be… I wish not to speak…

Fire Keeper: Ashen one, my thanks for the eyes thou'st given. But Fire Keepers are not meant to have eyes. It is forbidden. These will reveal, through a sliver of light, frightful images of betrayal. A world without fire. Ashen one, is this truly thy wish?

In DS2 the Firekeepers are also apparently blind, but do not speak of their lack of eyesight.

So, after collecting all of this, it appears that the Firekeepers are initially revered, then "decommissioned", and potentially forgotten altogether, going from simply serving as maintainers of the Fire for others; to serving the fire itself.

So what was/is the Firekeeper's role in the Dark Souls lore? Who do they serve (The Fire? The Cycle? The Lord Souls?) how are they linked to the Fire (are they a variation of the Undead that simply has a stronger bond to the Fire itself?) and what is it about the "Impurity" of the sense of sight or the ability of speech?