I get the feeling the purpose of the "rival" mentality has evolved over time. At first was just a simple case of competition and nothing more. However I think at this point it's become a defense of some sorts. Perhaps she's afraid/hesitant to admit she likes Kei for whatever reason. So she could be using the rival thing to avoid the issue.
Still I guess a lot of it depends on when the transition happened. When did Hikari goes from simply seeing Kei as a rival and instead started to see him as something more? That can make quite a bit of difference in terms of what will happen in the remaining episodes.
Kei and Hikari is one of those it's just a matter of time things. That much I'm sure of. The question is "How long will it take?". The problem is more with Hikari than Kei since she's too focused on competition.
For me episode 18 and 19, until now, are my favorites. but it's a shame that the relationship with hikari and takishima still didn't progress...do you think they're gonna be together? I heard they won't
I'll just say for the record yes I do like Special A. The first few episodes gave me worry because it could have gone down a terrible direction. However as it progressed it proved it was planning to go down the right direction. Episode 19 for me is the best so far.