Light Yagami's death in the manga occurs because Ryuk, the shinigami who gave him the Death Note, decides to end his life. It's a result of Light's descent into madness and his attempts to evade justice.
I'm not sure specifically which 'Kazuma Yagami' in a light novel as the name is not widely known in extremely popular mainstream light novels. It could be a character from a relatively niche or new light novel series.
First, understand Light's character. He's intelligent, calculating, and has a strong sense of justice (although his methods are extreme). When making him possessive, show how this new trait fits into his existing personality. For the OC, create a backstory that gives reasons for Light to be possessive. Maybe the OC has some information that Light needs. Then, focus on the relationship development. Use dialogue and internal thoughts to show how the possessive nature affects both characters. For example, Light might say things that are subtly controlling to the OC, and the OC might feel conflicted about it.
Since I don't know the specific Kazuma Yagami light novel, I can't determine the main themes precisely. However, common themes in light novels include friendship, adventure, love, and self - discovery.
Some fanfictions focus on Light's relationships. For example, how his relationship with his family, especially his father, might have changed if things had gone differently. There could also be fanfictions that explore a romantic relationship for Light, which is quite different from his single - minded pursuit of his 'new world' in the original story.
One idea could be that Light Yagami discovers the wizarding world. He might see the use of magic as a way to enforce his idea of justice more effectively. For example, he could use spells to secretly observe people's actions and then use the Death Note on those he deems as criminals in the wizarding world. However, he would soon face challenges as the wizarding laws and ethics are different from what he's used to in his own world.
Ron might be a bit more conflicted. On one hand, he would understand the need to get rid of really bad people. But on the other hand, he would be put off by the cold - blooded nature of Light's actions. He would probably say something like 'There's got to be a better way than just killing them off without a second thought.'
You can start by looking on fanfiction websites such as Archive of Our Own (AO3). It has a large collection of fan - created works, including many for Death Note and Light Yagami. Fanfiction.net is also a well - known platform where you might find some great Light Yagami fanfiction.
In some fanfictions, fem Harry Potter's magic can be used to counteract the supernatural elements that Light Yagami might face due to the Death Note. For example, she could use spells to block the Shinigami from interfering. Light's strategic mind, on the other hand, can help Harry plan better in her battles against the dark forces in her own world.
First, focus on the personalities of the characters. For Light, his intelligence, his sense of justice (however warped it may be), and his calculating nature are key aspects. For the reader character, give them unique traits that can interact with Light's. For example, if the reader is empathetic, it can create a contrast to Light's cold - heartedness. Avoid any sexual innuendos or adult - themed situations. Instead, build a relationship based on mutual respect, admiration, or even rivalry.