Didn't he confess to her when he kissed her on those steps?What was that about?
Their first kiss was accidental and this time, Ken kissed her for real. I think Rei saw it as teasing, which is why she slapped him then laughed. Also your comment is spoiler isn't it?
ikr I thought he would've been fine with Hasebe marrying Mikoto in episode 3, guess he really is a psycho
if you're asking for todome no parallel's ending, i'd say sure it is. they could've made it better but still okay. episode 10 is the only episode where they continue where they left off