When writing a Noctis OC fanfiction, it's important to keep the tone consistent with the original. If the original is dark and gritty, your story should follow suit. Research the existing characters well. Noctis has his own personality and goals. Your OC should complement or contrast with him in an interesting way. For example, if Noctis is more reserved, your OC could be outgoing and energetic. Also, edit your work carefully. Check for grammar and spelling mistakes, and make sure the story flows well from one scene to the next.
First, you need to develop your OC well. Give them a detailed backstory, personality traits, and motives. For example, if your OC is a warrior, explain why they became a warrior. Then, make sure your OC fits into the world of Noctis. Know the lore of the game or story. Also, create interesting interactions between your OC and Noctis and his companions. Maybe your OC has a different view on the prophecy that Noctis is a part of, which can lead to some great debates and character development.