I am usually okay with tossing aside backstory that is irrelevant to the main plot, but I seem to be having trouble letting go of a particular one that I think explains the character and his actions very well yet has virtually nothing to do with the plot (except as a motivator for something he did but that could easily be replaced with something else). It's plot-driven thriller with an unreliable narrator (I know, risky, but it's the best fit for the story I'm writing - I usually prefer third person) and the key source of his unreliability is 1) his utter ignorance to what is really going on behind the curtains in the story world, and 2) his tendency to fabricate lies for himself in order to cope with certain things, even if said lie is to the extreme. I guess you could say his personality borders neurotic. The biggest lie he fabricates is the backstory I'm struggling over whether to include or not. It would be the largest indicator of his unreliability, and would act as a large turning point mid novel when another character points out the truth. Basically, he married his college girlfriend, Ariel, and it started out as a very happy marriage. But soon things started to break apart and the marriage ended badly. The MC was so distraught over the failed marriage that he came up with this fabricated lie to convince himself and others that Ariel passed away, sadly ending a happy marriage. He soon begins to believe this lie. I like how the backstory develops his character, but, like I said, it's a plot-driven thriller and it would only develop his character, not the plot. I hear from some sources that character developing backstory is okay, while from others that it should only be plot relevant, so I guess this is more of an opinion question. I feel like if I leave it out it would take a big chunk out of his character, but if I leave it in I'm afraid it would interrupt the plot. The plan would be for him to periodically allude to his wife to the reader, not giving away too many details but implying that she has passed until another character tries to snap him out of it.