I think you have become slightly confused between the two main characters of this excellent poem. Your question implies that there are three characters, but do not forget that the speaker and the narrator are actually the same character, and, in fact, there are only two central characters in this excellent example of a dramatic monologue.
The principal character is of course the speaker of this poem, who is shown to be a character who is obsessed with his lover, Porphyria, and quite literally insane. He is in love with Porphyria, but reacts violently when told that Porphyria is not able to be with him because of the way she would have to reject society and the demands that it placed on her. Note how he reacts to possessing her utterly for one last time before she will leave him forever:
That moment she was mine, mine, fair,
Perfectly pure and good: I found
A thing to do, and all her hair
In one long yellow string I wound
Three times her little throat around,
And strangled her. No pain felt she;
I am quite sure she felt no pain.
Possessing her utterly is something that he never wants to end, and so he decides to kill her so that he can gain eternal possession of her and she can be "his" forever. Note the way that he deceives himself in the last two lines of the above quote into thinking that she felt no pain when she was killed. The narrator is therefore an incredibly disturbed individual.
The only other character to feature in this poem is Porphyria, who is blonde and beautiful, but also who rejects her lover and her emotions for the sake of society and the expectations placed upon her. However, at various points, her emotions overpower her and force her to meet with her lover, and this is what brings her to the speaker of the poem on this fateful night, when her inability to be with him forever will ironically lead him to kill her in order to possess her.