"La Belle Dame Sans Merci" is in the form of a dialogue between two speakers. The first is the unnamed speaker who comes across a sick, sad knight and pesters him with questions for the first three stanzas. Stanzas 4-12 are the knight's response. There aren't any quotation marks to tip you off to the change in speaker, so you have to pay attention to notice that the "I" of stanzas 4-12 is different from the "I" of stanzas 1-3.
Having cleared that up, what kind of speaker is the unnamed person who finds the knight? Is it supposed to be the poet himself? Or is it supposed to be the reader? Or an anonymous passerby? We don't know a lot about the speaker, but we can make some guesses based on what he says in those first three stanzas. Whoever it is, he uses old-fashioned language typical of medieval romances (like "thee" and "woe-begone"). He's also very sensitive to the changes of the seasons – he doesn't just say "Hey, knight, why are you hanging out by the lake when it's so cold and dreary?" He uses a lot of rich imagery to describe the seasons. He's also very perceptive of the knight's physical and emotional state. The speaker notices that the knight is "haggard" and depressed-looking, and seems to have a fever.
The knight uses the same kind of language that the original speaker used. In fact, it's difficult to tell that it's another speaker. Usually, when poets or novelists write dialogue, they try to differentiate between the different speakers by making them sound different. But not so in "La Belle Dame Sans Merci." One possible reason for this might have to do with the ballad tradition that Keats was mimicking. Ballads are like folk songs, and they could either be sung or recited by one person, or could be divided up between different people. Having the knight and the original speaker sound the same could just be Keats's way of creating a sense of unity to the poem.
Another possible reason is that the knight is just a figment of the original speaker's imagination. After all, what happens to him is incredible, and the dream sequence at the end emphasizes the possibility of illusion. If that's the case, the knight would never have spoken, and it would have been the same speaker for the entire poem. There isn't an obvious answer to this question – critics and readers still debate the meaning of the poem today.