To develop a believable Shinigami OC, start with their motivation. What drives them? It could be something as simple as a desire for more apples (since Shinigami love apples), or it could be a more complex reason like a longing for a different kind of existence. Then, look at their physical appearance. It doesn't have to be the standard Shinigami look. You can give them unique features that reflect their personality. For example, if they're mischievous, they could have a sly glint in their eyes or a crooked smile. Also, think about how they speak. Their dialogue should match their character. If they're arrogant, their words should convey that.
First, consider their backstory. A Shinigami's past can greatly influence their present behavior. For example, if they were once a human who had a tragic end, it could shape their view on death and how they interact with living beings. Second, think about their personality traits. Are they cold and calculating like most Shinigami, or do they have a softer side? Maybe they're curious about humans in a way that other Shinigami aren't. Third, their relationships with other characters matter. Do they have a rival among the Shinigami? Or perhaps a mentor? These relationships can add depth to the character.