The answer to the blank is: not credible.
A not credible, or unreliable narrator, is usually a first person narrator whose credibility is seriously put into question. The reasons for this can be as simple as contrasting his actions with his words and thoughts and realizing that they don´t match. Other, perhaps more interesting ways to make use of an unreliable narrator, would be to follows Poe´s example, and making the narrator claim from the start that he is believed to be extremely mentally ill, or that he is, or has been, from the start, mentally ill, which compromise his perspective.