Fuma Monou is a sixteen year old highschool student, and is Kotori's protective older brother.
Fuma had no plot development -_- He was very one-sided and it didn't show anything of his developing insanity or the reasons for his killing spree. Indeed, he literally just went insane halfway through the story. I think it might be something to do with being told he is Kamui. He kills half his own side, as well as the people he is supposed to be killing. He killed Kusanagi purely for laying a hand on him. He also indirectly caused Satsuki's death, as well as killing Sorata, Yuuto, Kanoe, Arashi and Kotori.
I thought he would be as big a jerk as he is in the anime, but I'm surprised to find he isn't. I, personally, don't like him very much, but I am fairly sure that that is the lingering remnant of the anime lodged in my mind. He comes across as far more protective of Kotori in the manga. Even the people he plays soccer with agree that he's very protective, taking her to and from school on his bike; they also agree that he comes across as cold. The neighbours describe him as stern, "but he's really quite kind", rescuing a little boy from a lake, and taking another neighbour to hospital when they were ill. All the girls at school adore him, even though he appears to show no interest in them, but even they agree that he acts very coolly. [Personally, it makes me wonder what they see in him O.o -Kachi] He seems almost hurt, however, when Kamui rebuffs him, first at the soccer pitch, then when Fuma finds him hurt and bleeding. It's with Kamui and Kotori that he shows his more caring, less stern side. He constantly tries to protect Kotori from everything, and tries to shield her from her memories of their mother, Saya, dying the way she did. When Kanoe reaches him, it seems almost inevitable what is like likely to happen, however, and by the end of book #6, 'Duet', it seems that he has started to change.
X Japan song:
Probably, 'I'll Kill You' is the one I think most of when I think of Fuma, although 'Stab Me In The Back' seems a little...apt, also.