So my main character's best friend acts as a supporting figure for him throughout the story but I don't want her to just exist for the sake of helping the MC. I'm having a hard time coming up with a deeper backstory for her because I don't wanna fall into stereotypes or cliches.
How to flesh out a supporting character?
group
I'd recommend treating her as if she's a main character for some time. I do this for some of my characters and it has worked pretty well. It's okay if you aren't able to put all of her backstory into the story because just having it can make her seem much more real.