She's cosplaying Aya, right?

I think it's her school girl uniform.

Then why is she wearing Pyonta? I guess maybe he's pulling bodyguard duty. Given Sanae's youth, beauty, and attire, kidnapping is a legitimate risk. Cue the hat-shaped mishaguji hopping off her head and either sprouting a body to punch the creeper out with, or a mass of tentacles to turn the tables.