The title suggests that the billionaire is a key character. He is likely to be a complex character, with his wealth and power shaping his actions in the search for his darling. The missing darling is also a main character, though we don't know much about this character yet. There could also be friends or family members of the billionaire who play important roles, either in helping him find the missing person or in dealing with the consequences of the disappearance.
Since I haven't read it, I can only guess. Obviously, the billionaire is one of the main characters. And his missing darling would be another main character. There might also be some sort of antagonist, like someone who has a vendetta against the billionaire and is involved in the disappearance of his darling.
In the 'billionaire's missing darling novel', the main characters are surely the billionaire himself. He is probably a character with a strong personality, used to getting what he wants. His missing darling is another crucial character. This character might be a symbol of love and innocence in the story. Additionally, there could be side characters such as the billionaire's business partners. They might be involved in the story in different ways. For example, some might offer their resources to help in the search, while others might try to take over the billionaire's business during his preoccupation with finding his darling.