I liked this story. Nobody was nice, but nobody was idiotically mean, though I dislike Shin way more than the other two.
In the end, they all have their flaws, but because Shiiba and Mitsuji are "normal" teenagers (eg because they have wishes like "I want to fit in", "I want to be liked", "I'm afraid to be cut off" etc), they weren"t able to express themselves honestly.
Shin, on the other hand, seems to be more perceptive and a "couldn't-give-a-damn" type of person. Somehow, I think he helped Mitsuji and Shiiba to grow up, even though it was all for his own benefit.