When I started, I just expected so much more. I didn't think that by the end of this manga that I would start disliking the two main leads and feel so passionate about the two supporting characters.
At first, I liked Riku, but as the series progressed, he turned into such an annoying static character for me. The main reason was that he was so indecisive for his feelings between the two girls. This trait continued throughout the whole series. First, Ao made the decision for him. Next, Kiyono made another decision for him. He never decides for himself and that honestly frustrates me.
Ao, on the other hand, just seems stagnant throughout the rest of the series. Her character never really develops after the first three volumes and becomes a little uninteresting. You know she's suffering and that's all you really get to know about her after a while.
No one really liked Kiyono much, but I thought she made sense in a way. She is deeply in love, but she was able to move on. This shows a change in her character and how she matures. Kai undergoes some changes also as you see more of him. Instead of being the cool neutral guy, he falls in love, has desires, gets angry, gets hurt, feels regret, feels pain, and earnestly tries to make the girl he is love with feel the same thing for him.
The supporting characters have so much more development than the main characters that I ended up caring more for them. This could explain why this manga ended on such a sour note for me.