The Billy Loomis of the first film and the one you find in character studies are almost different people, and the fanfiction that leans into that transformation through a reader-insert lens can be unexpectedly thorough. A lot of fics I've read start with the established persona—the charming, aloof boyfriend from the surface—and then use the reader as a catalyst or a mirror to dissect the fractures. Growth isn't always redemption, which is crucial; sometimes it's just a deeper, more horrifying understanding of his narcissism and performative nature. The reader's trust becomes the stage for his manipulations, and the 'growth' is the slow, dreadful realization of the performance, for both the reader character and, by extension, the audience. I'm less convinced by stories that try to reform him into a straightforward romantic hero—it feels antithetical to his core. The more compelling trajectory is the reader's own growth from naivete to survival, with Billy's character 'growing' only in the sense of his mask slipping completely, revealing the static, violent core underneath. It's a study in pathology, not healing.
Of course, some authors take the alternate route of 'what if' scenarios—what if something disrupted his path to violence earlier? That's where you see attempts at actual moral growth, but they're tricky to pull off without feeling contrived. The most believable ones frame it as a constant, fragile struggle against his ingrained nature, with the reader relationship as a tether that's always on the verge of snapping. The tension there isn't about whether he'll become a good guy, but whether the performance of being one will hold for one more day. That's its own kind of bleak character progression.