Rin's initial cold apprehensiveness does not endear her to me, but Himari and Yuu being able to make her laugh and relax around them helps. Considering that Shinji swore not to make any moves on Rin and likely doesn't want his ex Himari back, I think we can expect a classic love triangle situation in the future for these three.
Bro I have no idea what actually happens. If you can't figure out that she's the girl from before, then I'm worried about you.
Sorry, but I think the way you worded it was misleading. You were stating that Yuu is going to go out with Rin, and the wording was like it's a fact, like it was taken either from the LN or from future chapters and not from speculation:
and he'll start going out her. Then short hair girl will kinda be cucked and try to get him back.
If this is speculation, better add "I think" or "I believe" to statements like that because otherwise it really does look like you're putting major spoiler info out there. I don't blame the other guy for thinking it was a spoiler.
We know from Chapter 1 that Rin is the childhood girl based on looks, but I'm sure we don't know yet at this point which girl is the one he's going to date.