Kazutaka Muraki is the primary antagonist in Yami no Matsuei. His angelic appearance and characteristics serve to contrast with his brutal nature. Muraki's psychological troubles appear to have begun in childhood with his mother and his half-brother Saki. Muraki's mother collected dolls, and she is shown as treating him as though he were a doll too. Muraki's love of dolls and his collecting of dolls is a motif throughout the manga and anime, paralleling what he does with real people. In the anime, it is suggested that Saki killed Muraki's parents when they were still children and also decided to kill him. However, in the manga, it is not clear what Saki's role was other than disrupting Muraki's childhood, and Muraki describes himself as his mother's murderer.
Whatever the circumstances, Saki was shot by one of the family's guards, and Muraki became obsessed with bringing back Saki in order to kill him himself. Thus, Muraki hopes to use Tsuzuki's body to bring back Saki. This subplot features prominently in the last 3 episodes of the anime and volumes 7 and 8 of the manga.
Muraki is an expert manipulator, fronting as a good doctor who laments over his inability to save lives, while hiding his private life as a serial killer. Muraki discredits his good "Dr. Jekyll" persona, contrasting himself to Pr. Satomi in the Kyoto Arc, episode 12.
Being a serial killer, Muraki has numerous victims, the most significant of which is Hisoka Kurosaki. After stripping the thirteen year old Hisoka naked, Muraki placed a curse on him. The curse, which was untraceable—partially due to Muraki's having wiped Hisoka's memory of the event—resembled a terminal illness, and Hisoka languished in a hospital for three years before eventually dying. Later, when Hisoka is a shinigami, Muraki forces the boy to recall the night that he cursed him.
Throughout the story, Muraki manipulates souls of the dead, often killing the people himself, in hopes of drawing the attention of the Shinigami, in particular, Tsuzuki. He is obsessed with Tsuzuki's body; both carnally and scientifically. In the manga it is clear what Muraki desires, but the anime had to censor such extremes, and so Muraki's advances toward Tsuzuki were shown as hints of sexual harassment. Later in the anime it is shown that Muraki desires Tsuzuki's body to revive his half-brother.
A respected physician, Muraki has many connections throughout Japan among powerful patrons, but in the anime and manga, he is mostly seen in the company of his close friend Oriya and his old teacher, Professor Satomi. Muraki also has a childhood sweetheart by the name of Ukyou, but very little is known about her, other than that she appears to draw evil spirits to her and that her health is poor.
Some readers believed that because of his different colored eyes, he may be a guardian of one of the four gates of GenSouKai (see Wakaba Kannuki). However, in the King of Swords arc (volume three in the manga), a scene where Tsuzuki knocks out the fake eye reveals that Muraki's right eye is not real and that it is mechanical. The origin and nature of Muraki's supernatural abilities remain an enigma: he is human, he is alive (not a Shinigami), yet he raises a dead girl to be a zombie, seals and opens Hisoka's memory by mere touch, controls creature spirits similar to Shikigami, enters Meifu by himself and teleports Tsuzuki to another location.