In "The Raven", what purpose does the raven serve in relationship to the narrator?

The raven that comes to visit the man in the middle of the
night serves as both a messenger and a source of evil. A black bird (with the color
black symbolizing evil), the raven's appearance alleviates the narrator's loneliness
somewhat, but the man's misinterpretation of the bird's words drives him to the brink of
madness. At first the narrator thinks it a "stately" bird, but he eventually recognizes
it as an unwanted visitor. The raven also represents death, and it serves as a reminder
that the narrator's lost love can never return to him. The raven also offers the man a
supernatural presence, with the bird's ability to clearly state the word "Nevermore."
Author Edgar Allan Poe relates that the bird serves to maintain the narrator's need for
"self-torture" and self-grieving.