What does 'underwritten" mean in the following sentence?

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)