I had thought about that as well, and even though I like the "cold" part of Rintarou, I actually liked the character development as it was quite smoothly done, I think (I dind't really how much he had changed until I read the manga again)
I can only agree with you, it was smoothly done.
Rereading this manga.. i notice that too.. he looks manlier too.. and he looks more like his brother
Maybe the author intended that... Maybe... Just maybe, sthe author wanted it to be oneshot and due to its response gotten more chapters but she decided that to make the story more suited to the way she wanted it to go, she has to change the character... Without the oeople noticing much... In other words, make the transition look natural.
If you compare chapter 1 Rintarou to the Rintarou of the last few chapters.... Ch.1 Rintarou is a bit tsundere, or something close to that? Like, he actually shows expressions of annoyance, irritation, etc. In the last few chapters (actually way before that), he has become this sweet mess of a guy, like, he's full of fluff and not a single negativity. Character development much? I kinda miss when he was a bit stubborn, like in the first few chaps. Anyway, I still love this manga. One of my faves.