Kotori Monou is a sixteen year old highschool student, and a dreamseer.
In the anime...Kotori did nothing. She seemed to exist solely as a plot device, to be used as anyone wished. Her kidnapping happens, and no one - not even her - notices until it's too late. She just is as weak as Kamui, without any true personality. Her 'insanity' is handled very badly in the anime. She just vanishes into herself, so to speak, after Kanoe shows her the future. The excuse is that she couldn't handle it, so she visits Kamui in his dreams, encased in this strange pink goo. Even when Fuma kills her, she seems to act very half-hearted about it.
After watching Kotori in the anime, I was prepared to dislike the manga girl with a loathing. But instead, I encountered a happy, chirpy character who, despite her illness (a weak heart) is determined to be happy and outgoing. When one of her friends plays rough and another one admonishes the first for it, Kotori tells them that she is fine and nothing will hurt her. She also seems to have a great and deep love for her brother, saying at one point that his soul is 'eloquent'. The boys at school all adore her, although Fuma is protective the the point that he will not let anyone near her. She also adores Kamui deeply, and when she dreams of his return is very happy. It almost seems that Fuma knows of her dreamseeing, as he seems rather surprised that she dreamed of Kamui's return. When Kamui rebuffs her at school she is hurt, and collapses at the base of the stairs. Yet when she wakes in hospital, she's sure that Kamui took her there, and still cares for her.
In the second book, 'Overture', she still always thinks of Kamui. When his nightmares cause a minor earth tremor, she panics and tries to find somewhere to put him where he'll be safe, but in the end shields him with her own body. However, when he wakes up, he immediately becomes nervous and flustered. It's this shy nervousness that makes Kotori so appealing. Later in the series, she sees Kamui in a tree and immediately becomes nervous. Mistaking Kamui's fear for her for something else, she panics even more, saying "I wasn't coming after... ...you! I mean-- I didn't... oh!" then falls over a pebble. Kamui immediately dives to her rescue, holding her to him in an effort to keep her upright. She panics again, and in her panic brings them both to the ground. Through both her and Kamui's flashbacks, we see how Kotori was always fearing for Kamui, even when she was a small child, and the lengths Kamui would go for her, even if she didn't want him to endanger himself. This is a very sweet section ^_^
X Japan song:
Again, 'Crucify My Love' is what I think of, due to Kamui's dreams.