I don’t know what kind of dumpster fire the characters have got themselves into, but the little gremlin inside me wants this to be even more confusing.
It probably is Kawashima. It's been established that they were childhood friends, but he doesn't remember it was her.
Is that dumb? Yes, but it fits with this being all about using as many silly tropes as possible. It's also still better than it being her sister, mother, or some new character.