If that girl's dad helped with tuition for both him and his sister then wouldn't there be financial records available to prove her existence?
From what I've understood so far it's like the person really never existed so probably any trace of her disappeared completely including any material evidence, otherwise he would've had some of her stuff left, like a birth certificate at home, written notebooks etc
If that's her dad it's about to get really awkward, maybe Sahara Gensei will start to limit testing now lol but I think not, he's like the senpai with those eyes, so someone who also got taken and "came back".
Speculating now, maybe all the nenokamis were people and some managed to gain some part of consciousness and are like senpai, since Hebizono was asking MC if they think the nenokamis have a heart and here comes another person like her. Maybe since it's a place where horny people are swallowed to, the nenokamis are people consumed by lust?
