Answer:
To have an unreliable narrator, one must have the ability to lie, and this may be the closest definition you can get. You can't put your faith in a narrator who isn't trustworthy to tell the whole story. He has to deceive the reader in one way or another. The simplest and most obvious solution is to make it a part of who he is.
Step-by-step explanation:
Teachers are smart, so change some words to avoid plagiarism. :)