As I haven't read the 'I Sold My Body to a God' novel, it's hard to say for sure. But likely, the protagonist who makes the deal to sell their body is a main character. And of course, the god to whom the body is sold would be a crucial character. There might also be other characters like friends or family of the protagonist who are affected by this decision and could be important in the story.
Not having read the 'I Sold My Body to a God' novel, we can only guess. The person who sells their body is bound to be a central character. This character might be someone who is desperate, perhaps in financial or emotional distress. The god, which is the recipient of the body, would also be a key character. There could be other supporting characters such as a mentor - like figure who tries to dissuade the protagonist from making the deal, or an antagonist who wants to use this situation for their own gain.
I don't know the main characters in this novel since I haven't read it. Maybe the one who sold their body is a main character, and there could be a god figure as another important character.