In Nicky Shearsby’s new psychological thriller, Green Monsters, the first-person narrator, Stacey Adams, makes no secret of her hatred (her word, not mine) for her married older sister, Emma. Emma is a successful businesswoman, lives in a huge house with her dishy husband Jason and toddler daughter, has a designer wardrobe, yada-yada-yada. Perfect, in other words.

Emma’s every remark is perceived as a subtle dig at Stacey’s lack of achievement, her lower status, all the ways she is less. Implications all the more piercing by being true. Stacey lives alone in a cramped apartment and squeaks by with work for a temp agency at a job she cares about not one little bit.

This is a book that, despite its strengths, has a number of significant challenges buried in the set-up described above. Stacey has an almost Manichean view of the world. People are unambiguously either bad (Emma) or good (herself). There’s no gray here.

Shearsby does a powerful job conveying Stacey’s obsessions. The book cover describes her as a “narcissistic psychopath”; however, no mental health professional makes that diagnosis. When, eventually, the plot requires a reversal of Stacy’s attitude, I had been so persuaded of her pathology, I doubted whether Stacey would be capable of any recalibration. If she’s truly a psychopath, it isn’t plausible to me that one day she would simply get past it.

Any story where the main character has a severe mental disorder faces difficulties. And, in Green Monsters is also the narrator Leaving aside that a character’s quirks could become tiresome to the reader, it can be almost too easy to predict their actions. (Of course she sleeps with her sister’s husband—not a spoiler, says so on the cover. Of course, he’ll pursue revenge to the ends of the earth.) Such characters, propelled by their pathology, typically have little control over their lives, and all the reader can do is watch their downward spiral. (By contrast, in Tana French’s Broken Harbor, the apparent schizophrenia of the main character’s younger sister is brilliantly portrayed and viewed not through her eyes, but his.)

This isn’t to say that all characters need to be “likeable,” far from it. But they do need dimension. What do they do all day? What do they value? What are they interested in, and is it something that makes the reader interested in them? I never had the impression Stacey was interested in anything other than her sister’s husband and their trysts.

In the right hands, with the right project, there are always exceptions to any general observations about writing. But I’ve read enough stories that take the point of view of a deranged serial killer (which, thankfully, Stacey is not) that I have seen how hard that is to pull off. If I were trying to distill the main lesson for me from reading Green Monsters, it would be to give my characters the kinds of lives that will keep readers interested even when they are monsters, green or otherwise.