Pay attention to the pacing of your story. Don't rush the relationship between your OC and Kakashi. Let it develop naturally. Use vivid descriptions to bring the scenes to life, whether it's a battle scene or a simple conversation. Also, dialogue is key. Make the conversations between your OC and Kakashi meaningful and reflective of their personalities. For instance, if your OC is sarcastic, their banter with Kakashi could be really fun to read.
Research the Naruto universe thoroughly. Know the rules, the other characters, and the overall atmosphere. When writing about Kakashi, stay true to his character. He's calm, intelligent, and has his own set of traumas. Incorporate your OC into this world in a believable way. You could also add some plot twists to keep the readers engaged, like the OC having a secret connection to an old enemy of Kakashi.