The narrator is not dead or invisible. The narrator is trying to reason with someone else. In the beginning the narrator is talking to the other person trying to get them to realize that he/she has been at that person's side since the beginning. Then the person looks into the mirror and the narrator realizes that his/her reflection isn't there. Then narrator then understands that he/she is not actually by his/her friend's side, but rather only trying to rationalize with his/her self that he/she has been a good friend. In the end the narrator was never actually next to the person, and that is why the person wouldn't respond with the narrator.
Sometimes I feel like we like to be passive friends where we are too afraid of getting involved that we end up not being supportive of our friends. Often at times we lie to one another and say that we are by their side, when in truth we are only saying that to make that person happy. In reality we aren't actually there for that person, but we tell that person that we are until even we believe we are. That's what this is about.