Does An Unreliable Narrator Need To Be an Asshole?

Narrators! You can't rely on them these days. They always let you down, even the ones who have five stars on Yelp. One time, I hired a narrator to move a sofa and they claimed they brought it to my house, but it was actually five blocks away. Freaking narrators.

You've probably heard, or even used, the phrase "unreliable narrator." But what is an unreliable narrator, and why do we keep them around even though they let us down constantly? 

I often hear two slightly contradictory things about the ...