The key elements start with understanding the source material. Know the rules of the Miss Kobayashi universe, like how dragons interact with humans and their own society. Your OC's design and concept are also important. Is it a dragon with a unique appearance? How does that affect their role? Then there's the emotional arc. If your OC is going through a journey of self - discovery, show their emotions evolving. For example, they might start off feeling lonely but gradually find a sense of belonging. Also, the cultural references in the story. Since the series has a certain Japanese cultural influence, you can either keep it consistent or add your own cultural elements in a way that blends well with the existing world.
One key element is character consistency. Your OC should fit into the world of Miss Kobayashi without feeling out of place. Their behavior, values, and abilities should be in line with the overall tone of the series. Another element is world - building. Even though it's fanfiction, you can expand on the existing world. For example, if your OC is from a far - off dragon land, describe what that place is like. Also, the relationship between your OC and the main characters is crucial. It should be believable and add something new to the story. For instance, if your OC is Kobayashi's new coworker, how does that change her daily interactions?