The narrator in the story is quite truthful and reliable as a narrator. In fact, he is also good at eliciting information from the Prefect to get the full picture of what has occurred before the story opens. When the Prefect firs enters their lodgings, the narrator asks him,
"And what is the difficulty now?" I asked. "Nothing more in the assassination way, I hope?"
We feel confident that he is truthful in part because this is not the first mystery that the unnamed narrator and his two friends have assisted the police Prefect to solve. In fact, when the story opens, the three men are sitting in a darkened library, relaxing with their pipes, and the narrator is thinking about a prior case that the three helped with, "Murder In The Rue Morgue."
We also are not intended to believe that the narrator has any ulterior motives to provide any red herrings in this case, if it were in his power to do so. He tells the details of the case and reveals how his friend Dupin solves the case step by step in a linear fashion. Overall, his motivation appears to be that he enjoys the puzzle of solving a mystery and admires watching how his friend’s mind works and learning about his friend’s superior powers of observation and mental problem-solving techniques. He appears to both want to learn more from his friend and to retell the narrative faithfully.
The only times in the story when he might seem less than truthful—and it does not stem from his desire to lie but from his bias towards the Prefect—is when he describes the Prefect and possibly exaggerates his flaws. He says,
“That is another of your odd notions,” said the Prefect, who had a fashion of calling every thing “odd” that was beyond his comprehension, and thus lived amid an absolute legion of “oddities.”