Fuuma's younger sister, a delicate child with a congenital heart condition, and a developing yumemi (dreamseer). She was Kamui's very close childhood friend, and despite his initial coldness when he returns to her life, remains steadfastly kind to him. In the TV series, her dream is to become an indigo dyesmith (in the style of Japan's Edo period), well aware of the commitment necessary, and she spends time in her school's library reading up on the subject.

Kotori was an important key to making Kamui choose one of the two sides.

She was destined to die by the second Kamui, the only thing that was left to fate was whether Kamui was going to kill her or Fuuma.

However, since Kamui decides that the only thing he wants is to protect her (and thus indirectly saying he will join the Seals to protect humanity so that Kotori won't die), the person that finally kills her is Fuuma.

The way she dies in the manga and in the XTV series varies a little. In the manga, she dies by being stabbed in the chest with a sword but her body ends up being dismembered when the wires that kept her tied to the cross slice her body to pieces (manga only).

In the manga Kamui keeps her severed head and buries it. In the TV series, her body isn't chopped to pieces and Kamui buries her entire body.

However, since Kotori began to obtain special seer abilities little before she dies, she opens a sort of psyquic link with Kakyou even after her sudden death.
Kotori Monou is Fuuma's younger sister, a delicate child with a congenital heart condition, and a developing yumemi (dreamseer). She was Kamui's very close childhood friend, and despite his initial coldness when he returns to her life, remains steadfastly kind to him. In the TV series, her dream is to become an indigo dyesmith (in the style of Japan's Edo period), well aware of the commitment necessary, and she spends time in her school's library reading up on the subject.

She remembers with horror the day her mother died as she gave birth to the first shinken (sacred sword); when she sees the same thing happen to Tokiko Magami, she loses her mind, able to communicate rationally only in the dreamscape. Before she reawakens she is killed by Fuuma as he becomes the Dark Kamui. Hinoto, however, tells Kamui that she was destined to die—that whichever boy became a Dragon of Earth would have killed her. In spirit form she stays with the Dragon of Earth dreamseer Kakyou Kuzuki for a time, encouraging and thanking him, and telling him that the future has not been determined yet.

In the movie, Kotori dies when the second Sacred Sword is removed from her body. When Fuuma first attempts to do it, entering the dreamscape with such a purpose, Kotori actually escapes with Kamui's help and reaches Hinoto's room where he's waiting, but ultimately Fuuma catches her and pulls the Sacred Sword out of his sister's belly with his bare hands, killing her much to Kamui and everyone else's horror (Arashi, Sorata and Hinoto are also there). Kamui takes her body to the Tokyo Tower just before the final fight.
(Source: Wikipedia)