I'm a not fan of love triangles, but I'm giving the mangaka the benefit of the doubt.
This manga has repeatedly introduced cliché plot moves and then swiftly broken or distorted them.
I hope we get some good growth out of this. Kurokawa's reaction was honest and real to me. I've felt that. It stinks.
The most refreshing thing that could happen for me might be for Kurokawa to actually develop a friendship with this new girl. That said, I still find the new character kind of irksome, but much less than I anticipated.
I agree that the transfer thing was clunky, regardless of the role this girl will play. None of the other characters have stuck around or been fleshed out enough for me to remember them. I wonder how long this character is going to last. Her tie to Kurokawa seems too important for her to just disappear like the rest, but so far this story seems only to have room for two.