Uhh that seemed pretty psychotic. Like, she only knew her as a little kid, they basically had no relationship, and all of a sudden she's putting a ring on her finger? What??
I'd get it if she was an old flame, but this girl is straight up manipulative on day 1, wtf?!
I think it's more that we don't know the extent of their relationship, it's probably going to be constructed retroactively, but your point still stands. Doing these things like an hour after "reuniting" is some insane pedal to the metal type shit.
In any case, I have some thoughts of my own about Hikaru, but I also think that it's so early that any and all thoughts would probably be more fueled by projection than by any established narrative or character traits. I'm mentioning this explicitly about myself, but also to others, so they can keep vigilant if they share a similar tendency to my own.
Overall, I like what the story develops, but in truth, it's probably going to be too heavy for me to handle, so I might not be able to keep following it. I wish you who do that the ride be as bumpy as you like it to be.