The speaker in this poem is not anyone in particular. It is an omniscient third party whose identity we cannot know. We have no hints as to who is speaking -- all we know is that the speaker can know the thoughts of the people who are actually present in the ballad.
The speaker is not particularly involved in the drama that unfolds in the poem. He or she simply stands back, aloof, and narrates. Since this poem is in ballad form, one can imagine a folk singer as the speaker.
The speaker in this poem is an unknown omniscient narrator.