The writing style matters a great deal. It should be engaging and flow well. The dialogue should feel natural, whether it's Zero's sharp retorts or the soft conversations between him and the reader. Also, a good 'Zero Kiryu x Reader' fanfiction often has a unique take on their relationship. It could explore a deep, romantic love, or a strong friendship that is slowly evolving into something more. It should make the reader feel invested in the story and the characters.
A good 'Zero Kiryu x Reader' fanfiction often has well - developed characters. The writer should capture Zero's complex personality, his brooding nature, and his internal struggles, while also making the reader character relatable. For example, the reader could have their own insecurities that Zero helps them with, or vice versa.
The plot is also crucial. It could be something that ties in with the original 'Vampire Knight' story, like a side adventure during the events of the series. Or it could be an AU (alternate universe) that gives a fresh take on their relationship, such as a modern - day setting where they meet in a coffee shop but still retain the essence of their characters from the original story. Good fanfiction also has a good pacing, not too rushed or too slow in developing the relationship between Zero and the reader.