Speaker
The poem's speaker is an individual dealing with the end of a relationship and the abandonment of a lover. He (or possibly she—though Neruda has noted that the speaker is a representation of the poet himself) claims to be able to write the saddest poetry, but appears too sad and bewildered to truly express his feelings. Instead, he spends the poem trying, and seemingly failing, to reconcile his past relationship with his current loneliness. He finds this especially difficult because, while his circumstances have changed greatly, his surroundings are the same as ever. The natural world, which seemed inviting when he was in love, feels lonely and forbidding, though nature itself is unchanged. This genuinely perplexes him, as do his own emotions—he can't figure out whether he's still in love or not. Overall, the speaker is deeply sad, but he's both too sad and too uncertain to be able to verbalize his feelings. Therefore, much of the poem's poignancy comes from his fumbling attempts to put his feelings into words.
The Lover
We don't know very much about the person who has broken the speaker's heart, primarily because she's no longer present. We see snatches of her, especially her "infinite eyes," which the speaker seems to feel a particular fondness for. But in general, she's an elusive figure. Her vagueness and absence suggest that the speaker's grief and sadness is, in a way, a more real and tangible presence for him than his lover is. Indeed, she's no longer even reachable: the speaker tries to make her hear his voice, but doesn't seem to succeed. We can access her only through his memory, and as he forgets her, she disappears from us too.
