I don't see the big deal for us readers. She doesn't give a crap about that guy. It's either because that moment triggered something (like a memory) or something to do with being "praised" by an older "brother" figure. It's not romantic, just validation. To Raika, it will of coarse be a big deal since she always hid that face from him, just like he does around her. I'm sure we won't have to wait long for the truth. Now if it actually is an out of nowhere crush then I will be disappointed.