Author 100% has a pee fetish which would turn me off the series normally if if it wasn't such a minor thing and the rest wasn't great. Guessing since they used boku that this new person was also a guy at one point, unless they used that to mess with Mahiro.
Oh shit piss, the author's fetish raises its ugly head again. Also, I'm guessing the mystery girl(?) is Mihari's acquaintances. Because I can't remember her ever saying she would keep her experiment a total secret.