Seina's a good girl, but she did tell Uka she was going to fight for her feelings for Kai, fair and square. She 's aware Kai's and Uka's feelings are mutual, and it won't be long before Kai makes it known to Uka what he feels for her. In this aspect, she's totally " not nice " , because it looks like she's okay to break another girl's heart..
I don't think we can even consider there being a fight since Serina already gave up after saying that (in the beach chapter I mean). She's aware that Uka also has feelings for Kai, but the BOTH of them aren't sure of Kai's feelings, as pointed out by Takamine, so let's not hate on the women :/
I dunno what yr thinking but Serina aint that type of typical shoujo manga rival okay, what did u mean break another girl's heart, she didnt even tell Uka that she still liked Kai and uka found out about it by herself, u call that "looks like shes okay to break another girls hear?" Wow dude, dont speak trash if u dunno bout what shoujo magas are .
I looooove this manga and I might get hate for this but I love Serina so much like wOW her characterization is AMAZING but not in a way that I ship her with Kai bcus she's amazing with or without him (´∀｀•)