She is also the only one that had some. Interesting dynamic going on being shy but still gutsy and forward about her feelings for Arata.
All the other girls are keeping shit secret because they aren't brave enough to be open about it and frankly... None of them had an arc more interesting that the little date Kotono had with Arata.
That's also part of the reason why this series has me hesitant, I'm almost sure the winner is gonna be the annoying not-twin tsundere sister, yet Kotono it's the clearly better sister
Honestly, I think Kotono is the worst sister. Not that she's a bad person, but that I don't think she's a well written character. She's the only sister who has nothing going on beyond her crush on her brother. Literally, the crux of her story arc was that all she has going for her is that she's cute. She's been crushing on Arata for years because he told her that's enough, and... well... I disagree with Arata there.
The others all have lives totally separate from the harem situation. Banri has her aspirations of becoming a nurse who helps kids the way she once experienced as well as her responsibilities as the oldest sister. Seiha is a super genius, top of her class, and everyone is watching to see what college she'll choose while she's struggling to figure out how to handle regular human emotions. Minami is the tomboy tennis star with the exceedingly bright personality. Ouka has student politics, track, swimming, top 5 level grades iirc, and a lot of people these days don't like tsundere, but I typically find it hilarious.
With Kotono on the other hand, there's just nothing there. She is the least interesting character to me because her story is explicitly that she has nothing going on beyond being cute. In a quite literal manner, she needs to get a life, so that she can develop as a person. The way Minami did during the whole tennis tournament arc. It'll be difficult to grow if her entire existence is wrapped up exclusively in Arata. So, I would really not enjoy it if she won. Personally, my favorites are Seiha and Minami, but I know the frontrunners to win are probably Ouka and Kotono.
Seiha - most interesting
Minami - most fun (also the sweetest)
Banri - most entertaining
Ouka - most average female rom-com lead
Kotono - most bland (seemingly by design, which is weird)