Muko Tajuujinkaku/Dark Muko

Dark Muko (闇無辜 Yami Muko) is the split personality of the original Muko, and known to others as the very embodiment of insanity.

Appearance
Dark Muko, being the alternate personality of Muko, bears the same appearance that Muko had at the time she died, that of a young girl, about 15, maybe 16 years in appearance. She has long blonde hair (later cut), and glasses. Muko seems to have a preference for Gothic Lolita clothing. She is constantly seen carrying her Zanpakutō, never letting it down, so it is unknown how she wears it when it is unused. Muko always smiles, even in the midst of battle, and has a near constant blush. Dark Muko's most defining feature however, are her eyes, which have glowing yellow pupils and black sclera, similar to an Inner Hollow. When she is about to take over, Muko's eyes first glaze over, and then transform. When she speaks, it is a combination of Muko's original voice, and her own, much more raspy voice.

Personality
Muko, already a disturbed individual, becomes truly insane when the darker aspects of her personality fully take over, having been called even by those in Hell as the living embodiment of insanity. Dark Muko takes Muko's love of fighting and amplifies it, Dark Muko's only desire is to fight, and she absolutely does not care who. Weaker opponents, stronger opponents, even opponents she has of chance of beating, are all fair game to her. Like Muko, Dark Muko seems to derive sexual pleasure from fighting, but, unique to Dark Muko, also murder. She also seems to be a bit of a masochist, stating that she enjoys the feeling of inflicting pain and having pain inflicted upon her. Unlike Muko, who prefers to kill her opponents "fresh", Dark Muko loves to drag the battle out, to get wounded and extend the opponent's suffering as much as she can. Dark Muko states she has no loyalties to anyone, allies with no one besides Muko. No soul is safe around her, as when Dark Muko is in control, the chances of everyone the area she is in being killed is very high. She and Muko know of each other's existence, and are able to communicate similar to a Visored and their Inner Hollow. Not surprisinly, Muko and Dark Muko get along very well, often hosting "inner world matches" with each other to see who is stronger. The current record is 100:2 in favor of Dark Muko.

Zanpakutō
Yūnagi (夕凪 Evening Calm) is the name of Muko's Zanpakutō. It takes the form of short sword and a dagger. There is no known sheath. A unique Zanpakutō, Dark Muko is the only one out of her and Muko capable of activating it's Shikai, not because Muko does not know the release command, but because the spirit of the Zanpakutō seems to favour Dark Muko more. This does not bother Muko in the slightest.
 * Shikai: Released with the command Immolate with Madness (マッドネスで焼き殺す Maddonesu de yaki korosu). After releasing, the dagger lengthens slightly and the long sword changes to a fully black katana, which occasionally lets off a dark aura.
 * Shikai Special Ability: Yūnagi's Shikai Special Ability is the manipulation of energy. While a durable and strong Zanpakutō in melee combat, Yūnagi is capable of unleashing several attacks that utilize black energy. These attacks are very powerful, and the more they are used, the stronger they become. However, the more Dark Muko fights while using Yūnagi, the more of her already fragile sanity slips. Eventually, her skin darkens and her eyes completely lose their pupils as she gives into the insanity, which increases all of her physical abilities though she lacks the reason to use them properly.