Not everyone develops romantic feelings early, so, in that regard, Koharu is still fine. The problem is what previous comments are complaining about: her wishy-washy attitude to someone who has been clear/pursuing her for years - which resulted in everything being taken as granted...
Unrequited love used to be a huge tag in shoujo, so I've already seen it develop in way too many differents ways, and this one is still quite tame. The hate would not exist if she had any stance on the matter, other than being 100% passive (the point everyone hates on her). In short, it is not her 'cold shoulder' that is a problem.
From this point onwards, the author could pick any route and people would still have conflicting opinions. Whatever Yousuke chooses, I'd just wish for him to be happy... it's sad how rare it is nowadays to see a ML pursue someone without coming out as a creep you'd feel disgusted off, and, in my opinion, the pursuit itself is the strongest point in this manga.
I do miss good unrequited love stories, where one of the leads gradually makes the other fall in love. The circumstances and choices can be captivating!