The narrator is an enigmatic character. One way to explain his role is that the narrator’s job is simply to narrate the story. We don’t know his name, which is representative of us knowing nothing about him at all. He really only exists in relation to the Ushers, and that relation is primarily as an outsider.
When the narrator first arrives at the house, he notes several times the isolation and closed-off nature of the Usher family. Their bloodline has no branches, Usher never leaves his house, he and his sister have a special connection, Madeline is Roderick’s only companion. The narrator is on the outside of whatever eerie relationship the Ushers' share. He is also on the outside of the eerie goings-on inside the house of Usher. When Madeline passes by, for example, she doesn’t even notice or acknowledge the narrator’s presence. This could be because she’s too far away, or it could be because the narrator is intentionally being established as an outsider. When Madeline rises from the dead, she again disregards the narrator completely. He doesn’t even really partake in the story; he is unable to affect Usher in any way despite several attempts to cure the man of his melancholy. The narrator basically just watches the tale unfold before his eyes. You can start to see why some people suspect that he is a fictional creation in one of Usher’s stories – see “What’s Up with the Epigraph” for more.
An alternative point of view is that the narrator is significantly more important than just being a narrator. He is Usher's childhood friend and shows an incredible level of compassion towards the entire creepy situation (to agree give Roderick weeks of his life in the first place is pretty interesting, given that he clearly did not feel close to Roderick in the beginning). The fact that Roderick calls on him for help is interesting as his "only personal friend." Perhaps he was reaching out to help himself? Or did he just want an audience for his spookiness? (Again, check out “What’s Up With the Epigraph.”) You can also think about the way Roderick forms a foil to the narrator, which we discuss in “Character Role ID.”