Surely I'm not the only one that doesn't trust this Reika chick right? After her comment about not wanting to meet in a busy place she's def hiding something.
She's hiding that she's an adult hanging out with/dating a high schooler lol. I don't think she has any malicious intentions per se, but this relationship was never going to work out and Reika should've ended it before it got to this point.
As for Himawari, we've barely seen her so it's hard to speculate. It could be jealousy or possessiveness, but it could also be something like "You said you loved me, but you fell in love with this other person immediately so did you ever even love me in the first place?" I can see being annoyed by that even if you don't return the other person's feelings.
Anyway, this continues to be possibly my favorite ongoing manga; it's so good at portraying the rough edges of human relationships, how even people who care each other don't really understand each other, how you can never truly communicate what you really want to -- but also how, despite that, our relationships are the most valuable things we have. It deserves a much higher readership than it's getting, but I suppose it does take a while to get into.