The key to good fan fiction

So I just started watching the Netflix original series “Stranger Things” and one of my favorite parts about the story is the relationship/friendship between Nancy and Jonathan. I’m a sucker for a potential love story within a story. Some would consider that a subplot. I’m so into these two characters and their potential to develop a romantic relationship that I was curious about what kind of fan fiction is out there for these two. There was one fan fic that I really liked because I think it stayed true to the characters. The author made the characters believable. I don’t know if it’s because that person took from an actual scene and just reimagined it, but I think that’s only part of it. Some of the other fan fics weren’t believable to me because I felt like they didn’t stay true to the character’s personalities. I feel like that’s the key to good fan fiction: staying true to the characters and their development.