Well this is one development the author pulled out of his ass
To be honest, I saw it coming a mile away (figuratively speaking), but then I'm binge-reading this (and mentally processing during and after each chapter) and I've been leery of Himari almost as soon as we saw her thoughts on extending one's youth. It sounded like she was yearning to be eternally young like Tsukasa is, back in
Ch.235. The conversation she's having with Asimo, just before it transitions to Tsukasa mulling over what Nasa said about her 'power'. The very end of the conversation is the top panel on
Pg.7 of Ch.235. Combine that part of her conversation with her earlier musings about the fleetingly short nature of human lives at the start of the same chapter while at a gravesite (probably by her father's grave), and you can read some very bad vibes off her. At least, I did.
And then at the end of the Tokyo Tower dates, just two chapters before this (
Ch.239, Pg.18), it was clear they were implying she had seen Tsukasa before, and I immediately suspected she had seen her at some point around her father's death. I about thought this might have confirmed it, but 7 years isn't nearly long enough ago, so now I'm curious what happened then, and I'm willing to see where it goes, since they've been laying dark past hints for a bit.
Her gaze on the chapter title splash arts are really unsettling, too. Or, rather, they've been gradually getting more unsettling every time. She's practically glaring at the reader in the splash art for this chapter.