The Reaper is a representative figure. How does she become this?

The girl in the field represents a great deal to the speaker.  The fact that she is working with the land, cutting grain, almost makes her a part of the earth.  It cannot be overlooked that it is a young girl ("Maiden" is used to describe her) who is working in tandem with the land, almost representing a mother Earth type of symbol.  She is an extension of the earth, and when she begins to sing, she represents a sense of "oneness" and unity with the earth for the song synthesizes her with the naturalness of the setting.  In the next two stanzas, the girl and her song represent a sense of wonderment for the speaker.  He does not know what the song means or what the girls is singing, but he realizes what it has done to him.  Hearing her song, and by extension her, represents a sense of emotional flight into a voyage of imagination.  The song impacts him as a "nightingale", allowing him to envision "old, unhappy, far off things" or "battles" or "natural loss and pain."  The song represents a sense of seeking emotional connection with the melancholy strain that allows the song to exist.  Finally, the reaper and her song represent memory in the speaker, for as the speaker leaves, there is the direct suggestion that the memories of tenderness linger in our minds and psyches more than any other type of reverie when we have witnessed and experienced moments like the speaker:  "The music in my heart I bore/ Long after it was heard no more."  This indicates to us that the reaper now represents memory of beauty and unity, the goals to which all individuals should aspire.  The Reaper and the experience represents how reality can be transformed from what is to what can be as the speaker has undergone a moment in time that has allowed a greater understanding of natural beauty and daily life.