The Raven, black, ominous, enigmatic, and isolated, is the symbol of grief and loss. It is the epitome of the compiled emotions of the narrator at the time of the entrance of the bird into his household. While the bird, itself, is ignorant of the exertions that he causes upon the psyche of the narrator, the emotional gravitas that the latter bestows upon the bird is the pivotal factor that transforms it into a symbolic token.
Every behavior exhibited by the bird is seen by the grieving man as a foreboding message, telling him that the pain and the grief will never ever go away. Such is the essence of mourning: an emotional constant of anxiety and sadness that does nothing but remind us how finite life can be, and how unfair.
But the Raven, sitting lonely on the placid bust, spoke only
That one word, as if his soul in that one word he did outpour.
Nothing further then he uttered- not a feather then he fluttered-
Till I scarcely more than muttered, "Other friends have flown before-
On the morrow he will leave me, as my hopes have flown before."
Then the bird said, "Nevermore."
Therefore, the Raven represents the corpus of sadness, grief, loss, and mourning that is eating away the soul of the main character. These are emotions that, like the raven itself, are also dark, and seem to want to stay put "on the placid bust of Pallas" of our hearts, forever.