Well, the title pretty much says it. I have a story planned out and am about to start writing it. My main character is a teenage girl who finds out she is a witch/werewolf hybrid (either I'll make her adopted or her parents hiding it from her). I want to write her as having bipolar disorder. Do you think a supernatural can have a mental illness? I know that e.g. vampires not but I am not sure about werewolves and witches. What do you think?
^^That. Think about how the disorder would affect her, make your representation consistent in your world and make her believably bipolar. But don't give time to anyone questioning the realism of your werewolf.
FYI lycanthropy itself was classed as a mental illness as early as the Byzantines: Lycanthropy is a status of melancholy Meaning at the same time misanthropy. You recognize the affected man Running around the tombs at night time Pale, dry, sad and careless of his appearance.
What I would do, unless you're going to have her being treated by a fantasy psychiatrist, is make her behave as if she's bipolar, but you don't need to formally diagnose it. That way nobody can grinch, but your character can still behave that way.
You are the owner of your own fantasy, so you can use it as you wish! Moreover, in my eyes, the wealth of a character is also measured by the different dimensions that characterize him: those that benefit him, as well as those that constitute his shadowy parts in him. The series The Boys on Netflix is particularly eloquent on this subject.
Of course they can. First, it's your story so you can do whatever you want with your characters. Secondly, there's no "rule" that says a person's human characteristics (in this case, a mental disorder) cannot transfer to their supernatural persona. It's been done before too. One example I can think of, there was an episode of Buffy the Vampire Slayer (season 3 I think) where Buffy's powers were stripped, and she became trapped in a house with a vampire who was intensely mentally ill. He was literally being fed medication to control his behavior. It actually made the character scarier and more intimidating because not only was he a superpowered vampire, but he was crazy too
So long as it isn't a direct metaphor, i'd go as far as saying supernatural characters are more believable/engrossing with an emotional/physical downside. Being a werewolf would suck regardless I suppose...