Not sure if you still want the answer but it seems like the apartment is the one where the three of them lived together before. In the authors ending note, she mentioned the girl is the one who gave Kai the briefcase with the info about Cys. As for his hair, I have no idea why it went back to blonde haha
ohhhh this helps a lot thank you very much for the reply!
Umm, I'm not sure, but I guess the hair color changed to show the passage of time (they were apart for a year) and Kai's hair is naturally blonde, so the black grew out. That's my theory ^__^
Oh i didnt know they were apart for a while. Should reread this manga sometime. Thanks for the input~
I didnt quite get the ending...was that apartment the apartment where Kai and Cys took that one photo? And was that girl the one that gave Kai the missions or was it kai in a wig? What did those missions have to do with the plot? And didnt kai dye his hair black? Why did it go back to blonde?