Kouhai's appearance was definitely planned. We've been getting hints of the angst coming from Mashiro's side since the beach episode, I believe. I think we haven't really grasped how insecure Mashiro is. Right now we're thinking, oh cute that she's so clingy because they're in the honeymoon phase of their relationship. But Mashiro has been warning Nagisa all this time that she's a needy greedy clinger that's probably more reminiscent of a boa constrictor than the cute Koala some of us have been thinking. She's been isolated due to her illness and Mashiro's quite possibly the first connection she's made so it's understandable but, now we're entering the college entrance arc which I know isn't really supposed to hit until next year but remember, Nagisa wants to enter the field of medicine in order to help Mashiro.
She's already talking about cram school so I imagine less time for dates and I think, I really hope I'm wrong, that they may be put in separate classes which will add tension to their relationship. I imagine nurse can put things in perspective about long distance relationships since she's probably dating Former President Nanjou but given her nature, she'll probably just make things worse. Uda's a more likely character to shed light on this predicament seeing as she's pretty much going to be living the same life. Her future GF Riri is school president and the actual candidate for the college arc since she's the 3rd year student.
So yes, all this conflict that can and will arise. They were always handled quickly with regards to our mains but will this approach change? This manga has always been on the more light hearted side so I'm wondering how it'll maintain that tone while facing very real world elements that often has adults pulling hair from their scalp.
Kouhai doesn't seem like a romantic rival. Feels like she's there to remind Nagisa of her more studious self. Or perhaps the opposite will happen and Nagisa and Mashiro's love will demonstrate there are more important things in life than studying.
I think Kouhai will turn out to be their adopted child in the end though.
Thanks for the translation! Really need this beautiful thing in official English so I can continue to support. Don't get me wrong, buying copies at Kinokuniya is cool and all but official language translations can help the popularity of this so I'm still hoping someone will pick it up!