Whether you're creating a fanfiction or an original, characters are of the highest importance. The way I see it, you can't have a plot without characters, because the plot happens to the characters, not the other way around. Your characters are living through your plot.

Develop your characters, make them complex. I figure this also applies to fanfictions as well, even though with fanfics you're using either real people or characters others have already created. It doesn't mean that you can't give them a backstory they don't even have for the sake of the plot.

As an example, Felix in my fanfiction Murderous Love Story. He's a psychopath, he was born that way, but because of his upbringing he turned into what others see as a monster or a serial killer. In a way he's actually a victim of his own life, and he'll even admit that he most likely never wanted to be how he is in that fanfiction, but his past kind of forced him to go down that path. He also has his fears. You probably already figured this one, but he fears losing his gang members, but he also has a fear of love. He fears falling in love with someone, because the last person he loved so much was murdered by his own father.

My point is, even if it's a fanfiction and your characters are based off real people or pre-existing characters, you can still make them complex characters; you can still create them in your own imagination 😊