>>70340872my two cents. Shion is actually quite shy (one of the many reasons she's so hard to find in the collab pokedex) and takes a while until her kusogaki engine kicks in.
I think you have a good idea of how to write her in a bratty way, but it could help a bit to start with her doing deadpan remarks/being skittish until she realizes you are a country bumpkin and that's where she starts feeling more comfortable and finally ends up being bratty.