The raven initially symbolizes what Poe describes as "mournful, never-ending remembrance." The narrator's sorrow over his lost love Lenore provides the impetus for his unusual conversation with the dark, strange bird. But the raven provides no comfort for the narrator, a broken man still nursing a broken heart.
It's noteworthy that the raven is perched on top of a statue of Pallas Athena, the goddess of wisdom. This gives its constant refrain of "Nevermore!" a significance that it would otherwise lack. Through the raven's unconscious expression of wisdom, the narrator realizes that Lenore has been lost forever, never to return. And the narrator comes to hate the raven for this. He so desperately wants to hold on to the illusion that Lenore will one day come back. The shattering of that illusion by the raven changes the symbolism of what the bird represents. In the narrator's fevered imagination, the raven has now come to represent evil itself:
"Prophet!" said I, “thing of evil!—prophet still, if bird or devil!"
At the same time, the narrator acknowledges that the raven's dark shadow hangs over his soul. It's going to be nigh impossible for him to expel the evil that's now entered into the very depths of his being.