The first reply, of sorts, that the speaker receives when he opens the door to discover the source of the "tapping at (his) chamber door" is actually the darkness: "Darkness there and nothing more." The darkness and the silence, offered by the raven, fundamentally represent the speaker's loss. His "sorrow for the lost Lenore" has left a darkness and an emptiness in his life, and so the darkness at the door is arguably the first reply that reminds him of Lenore.
The raven's first spoken reply is simply an echo of the speaker's "Lenore?" The raven is in this sense a reflection or a part of the speaker, manifest in external form.
The next reply offered by the raven is to perch "upon a bust of Pallas" and sit there in silence. The silence again perhaps reminds the speaker of Lenore because it reminds him of what he has been left with now that she has gone: "stillness," "darkness," and emptiness.
When asked for its name, the raven replies "Nevermore," and this one word is its only verbal response for the rest of the poem. This word of course reminds the speaker of his "lost Lenore," because he is aware that she will never again return to him, that he will never be able to forget her, and that her absence means that he will never again be able to fill the darkness that he has been left with.