I might prefer to see 'under-written' there, to avoid confusion with 'underwritten' as used in the insurance business. It's the kind of word familiar to movie buffs and people who read reviews in e.g. Variety:
"With multiple authors but no clear voice, the clumsy “Mulan” script
often puts plot above character, depriving Mulan of a robust
personality. Defined by her determination, she mostly keeps to
herself, which deprives her of meaningful human relationships during
the mid-section of the film. Underwritten as she is, Mulan is handily
upstaged by Xianniang (Gong Li), a powerful sorceress invented for the
film who gives Mulan a strong female adversary. "
Mulan (Variety review)