Once you notice the piss thing, you just never unnotice it. It's even in chapter 22.5 that COG mentioned.
With that out of the way, this is definitely Mihari's senpai's younger brother/sister. Is TF'ing going to become Japan's number one way of dealing with NEETs from now on?
@Magneticmagnet Mihari's confused on the last page about why Mahiro is upset, so she probably doesn't know Nayuta.
If Nayuta is a NEET related to Mihari's senpai, it could explain why her senpai was doing that research in the first place: she also wanted to rehabilitate her sibling by age-regressing (and possibly gender-changing) them. Of course that would mean Mahiro was a guinea pig...