There are two opinions on the construction of La Belle Dame sans Merci by John Keats. The first is that there is an unidentified observer speaking to a knight lying on a hillside; but this view embodies some illogicality that is not neatly done away with. The second version is that the knight himself is speaking throughout as a sort of self-observing, self-questioning narrator.
The first view, which requires someone external to the story being at hand to observe the knight on the hillside, presents a problem in logic because if one were really to come across a knight lying on a cold hillside in winter, cold and with a lily on his brow and the color gone from his complexion, one most likely wouldn't politely question said knight. If one did, one certainly wouldn't expect an answer because the description of the knight embodies the poetic description of a corpse. If this is the case, a second problem in logicality regarding an external observer arises in that the knight does indeed answer, leading some to exclaim, "Man, if he can answer, why can't he just move the lily and get up and go?"
Keats was paramountly logical as well as poetical, devising intracate imagery and symbols, and the second view of the poem capitalizes on both these skills. In the second view, the knight is observing himself--what we might describe today as an out-of-body experience--and his questioning is rhetorical, not genuine. This is because as himself, he already knows the answers. In this version of the speaker's identity, Keats has employed a brilliant device to tell a tragic tale of a knight's demise through the manipulations of a life-devouring magical faery.
To address your question, according to the first view, the speaker in Stanzas I - III is the unnamed observer who has come across the wretched knight lying in the cold on the hillside. In this version, the knight takes over the speaking in Stanza IV and tells his tale in response to the observer's questions.
In the second view of the structure of La Belle Dame, the speaker in Stanzas I - III is the knight himself who is engaging in rhetorical self-observation and self-inquiry akin to the contemporary question "How did I get into this mess?" with the added twist of a ghostly or astral observer. In this view, Stanzas IV - XII continue with the knight speaking as he relates his self-observing narrative of his own tragic story of deceitful love.
Considering that Keats believed he would die in a couple of years (again, logical after having nursed his mother and his brother through tuberculosis to their own sad deaths), this poem may take on a different nature, symbolically placing Keats himself as the knight and the betraying faery as life itself, full of hope and promises of love all too shortly lived.
Personally, I believe that the first speaker, or narator is key to the whole poem.
In order to suggest a physical and recognisable identity for this figure, we must first look at the surroundings, and establish to where the poem is actually being set.
Naturally, we must re-read the poem several times, as I am sure John keats intended, in order to raise a logical opinion.
The setting is simple, a cold, lifeless hill. To add to this view of a lifeless plain, we must look at the way the narrator talks:
'no birds sing' and 'squirrels granary is full' suggest that any form of life, apart from the knight and his mysterious companion, is void.
This is an imminant suggestion of death. Perhaps the Knight has joined the Kings in his dream?
The setting and mood seem to reflect this view. Futhermore, if we look at death in a philosophical sense, there are many descriptions which imply that there is a long waiting after death. Certainly in Buddhism, and other religions, [In Catholicism you can look at purgatory] there is a suggestion of a wait before you can conitinue to the afterlife. A widespread analogy is that the existance of ghosts, is down to lost spirits waiting before they can continue the journey of death. This would explain the knight's "paley loitering," aswell as the king's appearances in hid dream.
Therefore, returning to the original topic; the first speaker.
Could this be the very physical form of death? Trying to take the knight onwards, yet the knight remains to "sojourn here."
This would not be a far-fetched guess as to the identity of him/ her.
This idea could be futher strengthened by looking at keat's other work. from looking at "Lamia" for example, we can see that keats loves Classic references, aspiring in Mythology in order to break the "age of reason" which was prodminant at the time.
This, coupled with the idea that romanticism and gothic literature are not far apart, would not be too otulandish to suggest that this theory could be correct.