Perhaps there's a best friend character. This person could be a mortal who is initially unaware of Nancy's demigod status. As the story progresses, they might get dragged into Nancy's world and have to deal with the consequences. There could also be a villainous demigod who is trying to stop Nancy from achieving her goals or is causing trouble in the demigod world in general.
Yes, Nancy Drew can be considered realistic fiction as it often involves situations and characters that could exist in real life, with some elements of mystery and adventure.
Nancy Mulligan has elements inspired by real life, but it's not a completely factual account. Songwriters often blend reality with imagination to create a compelling narrative.
Nancy Drew is definitely fiction. The adventures and cases she solves are all created by the author's imagination. It's not based on real events or people.
Sid and Nancy is indeed rooted in truth. It portrays the real-life relationship and events involving them, though with a touch of artistic interpretation to make it more engaging for the audience.