Well, it could be a story where the male OC is a detective trying to figure out the mystery of the Death Note, and Kyra gets involved in some way. Maybe she has some secret knowledge about it.
First, you need to establish the character of the male OC. Decide on his personality, his background, and his goals. For example, if he's a student, his goal could be to expose the secrets of the Death Note to the world. Then, introduce Kyra in a way that makes her relevant to the male OC's story. Maybe she's a witness to a Death Note - related crime.
Ryuk would probably just be amused. He is a Shinigami who has seen a lot in his existence. Fanfiction is just another strange human creation to him. He might chuckle at some of the more outlandish ideas in the fanfictions, like a character having superpowers beyond the Death Note.
Misa would likely be excited. She is a big fan of Light and the whole 'Kira' concept. In fanfiction, if there are scenarios where she has more power or a closer relationship with Light, she would be all for it. However, she might also get jealous if there are other female characters getting too close to Light in the fanfictions.
Oh, for sure! Death Note is considered a masterpiece in the manga world. Its unique concept and the way it unfolds make it a must-read. The characters' development and the moral dilemmas presented make it stand out from the crowd.