Ah yes, the bestie stereotype. There are a few of them, like the boy-crazy pal to your blossoming-flower protagonist. Try this: Imagine sidelining your protagonist and giving Bestie the ball. It’s her book now. Write scenes with her as the lead. What new traits, interests, flaws, and goals would she reveal when it’s all on her shoulders? If she’s not revealing any, push her harder. Ask her questions with no right answer and see which shade of gray she picks. Move her to a new scenario, or even to someone else’s book. Write a pitch for a novel about her and her problems. Apply what you learn to your original story. Let her motives and distractions show up. People tend to be self-interested, so besties shouldn’t be all about the protagonist. Bestie would still reflect, amplify, or provoke your protagonist, but a separate life would be evident. She’ll be a richer, unique character—and her contribution to your star’s arc will be stronger.
This is great advice. I’ve just started my second book for my MG. This allows me to develop the “Bestie” a little more.