@LauraTimothy93 I have an idea. Maybe the oc could be an assistant and like a daughter of buster moon.
She was adopted by ms Crawley when her parents either died or she never knew them as she was raised in an orphanage, music being her own sense of comfort.
She is introverted when you first meet her and when she sings she becomes extroverted.
She becomes a comfort and friend to everyone in the cast.
In the first movie, she is an assistant that helps Ms Crawley most of the time and when she meets Johnny she lobes his voice and helps him with his piano lessons, maybe singing with him.
Songs could be " just wanna be with you" from hsm3.
Her songs could be in the movie could be a duet with Johnny with I'm still standing or "waiting for a girl like you" from rock of the ages or "when there was me and you" from hsm ( I had Vanessa hudgens in my head for her singing voice and speaking voice). Another duet be with meena like " I'm looking out for you" from mlp next generation movie or sing the latter end of hallelujah with her when she finds meena.
In the second movie she could do the beginning of let's go crazy and sing with Johnny and on the chorus where rosita would sing with him.
Their song in the outer space show could be "rewrite the stars" or " right here right now" from hsm3.
They are like the romeo and Juliet of the two planets of War and Hope.
Those are some ideas, I have looked at other sing fanfics that use these in their work so hopefully this will give you some inspiration xxxx