Kōga Kuchiki

is a Shinigami introduced in the anime-exclusive Zanpakutō Unknown Tales arc. He is a former 3rd Seat officer Shinigami of the 6th Division who once revolted against the Seireitei. He was also a member of the Kuchiki noble family, having married into it.

Appearance
Kōga is a tall and lean well-built Shinigami with green eyes, black hair with a long braided pony-tail of red hair, and bangs that hung on the right side of his face to frame it. Back during his days as a Shinigami of the 6th Division, in addition to a standard Shinigami attire, Kōga also wears a long red scarf, light purple obi sash, and a beaded-necklace with a pendant. He also wore an intricate white headpiece called kenseikan on the left side of his head to symbolize his nobility with red tufts of hair running through it (which he later cut off to relinquish his status as a Kuchiki). While originally his appearance was youthful and fit, after several hundred years sealed away, his physical form has been reduced to a withered-old version of his former self.

Personality
In the past, Kōga was a man with a true sense of duty and a commitment to making his family proud. He always tried to remain humble and serious to situations at hand. On the inside however, Kōga was a man who deeply desired to be acknowledged for his abilities and hard work. After realizing that his life as he knew it was gone forever, he had a mental-breakdown, causing him to become consumed by his anger and desire to stand in greatness, leading him to become immensely arrogant in his power to the point of viewing all others as worthless, even his own Zanpakutō, Muramasa.

History
Several hundred years before the main storyline, Kōga was welcomed into the Kuchiki family as the son-in-law of the Ginrei Kuchiki. He soon gained much respect throughout Soul Society. Unfortunately, the one person Kōga wanted to be acknowledged by, his father-in-law Ginrei, always looked at him with indifference and put him through tests. The only person to ever comfort Kōga at all was his Zanpakutō Muramasa. Kōga later became a powerful ally in a civil war. When it became clear that he may very well be the key to winning the war, the enemies, which included three elites, came up with a plan to frame him as a traitor. He was later freed by Muramasa who inspired him to take control of his life again. Kōga then did so by killing the elites who framed him. But it was only after he did so that he realized he made a mistake. Realizing his life was forever ruined, he went insane and decided that everyone was jealous of his powers, thereby meaning he had to crush all others to stand tall in the world again and soon revolted against the Seireitei. As he killed more people, his instability grew, soon even abusing Muramasa when he was anything less than absolutely obedient. This disharmony between them gradually caused Kōga to lose his ability to communicate with and even access Muramasa's power. Genryūsai Shigekuni Yamamoto and Ginrei Kuchiki were eventually able to seal Kōga away when he lost total access to Muramasa's power. Kōga's revolt was never recorded in the Soul Society's history.

Zanpakutō Unknown Tales arc (anime only)
Note: Events occurring in this arc are only in the anime and do not constitute canon material.

Kōga was sealed away, but his Zanpakutō Muramasa eventually resurfaces and begins to initiate a Zanpakutō rebellion. When Ichigo Kurosaki encounters Muramasa, he states that he already killed his master. However, after Muramasa gains control over Ryūjin Jakka, Genryūsai Shigekuni Yamamoto reveals that Muramasa is planning to break Kōga's seal. He also states that Muramasa is heading to Karakura Town, where Kōga was sealed.

Muramasa eventually arrives at the location where Kōga was sealed, but comes into conflict with Orihime Inoue, Uryū Ishida, Yasutora Sado, and Rukia Kuchiki. He ultimately succeeds in breaking Kōga's seal. As Ichigo arrives to stop him, Kōga begins to awaken. Though Muramasa is ecstatic that his master is finally free, Kōga immediately stabs him in anger, leaving a shocked Muramasa to ask him why. Believing that Muramasa abandoned him before being sealed, Kōga decides to reinforce his status as master by making Muramasa pay with his life. However, Byakuya appears, stopping Kōga's strike and scolding him.

Byakuya reveals that he pretended to betray Soul Society so he could put the shame of Kōga to rest. After learning that Byakuya is the current head of the Kuchiki clan, Kōga becomes determined to destroy his association with the Kuchiki clan by killing its leader. Muramasa, despite being attacked by his master, insisted that Kōga use his power. Kōga arrogantly instead destroys the remainder of his sword, casting Muramasa away completely. Kōga retrieves one of the blades that sealed him and engages Byakuya, fighting him on even grounds. Kōga turns the battle to his favor by using his illusionary powers to confuse Byakuya. To even the odds, Senbonzakura combines his power with Byakuya. They then used their Bankai's Senkei form to counter the illusions. Shocked that Byakuya could still fight him as an equal, Kōga shouts that it is impossible for them to be so powerful. Byakuya merely says that it is because he fights as one with his Zanpakutō, thus allowing him to achieve his greatest strength. Kōga, on the other hand, cast away his full power to satisfy his ego. Refusing to listen to reason, Kōga is determined to finish the battle, forcing both warriors to go for a final clash.

Powers & Abilities
Master Swordsmanship Specialist: Kōga was an immensely powerful combatant, even with only his sealed Zanpakutō. Back when he was only a 3rd seat officer, he was able to easily fight at captain-level and take down a large group of enemies attacking him at once. His skills are very versatile, able to use various bladed weapons with high proficiency.

Flash Step Master: Kōga has demonstrated highly proficient skill in Flash Step, able to keep on equal footing with Byakuya Kuchiki, one of the most skilled masters. In battle, Kōga has shown great enough reflexes to dodge point-blank sword attacks unscathed.

Kidō Master: Kōga's skill in Kidō is immense, able to use various forms of spells with great power and skill; all at the same time without incantation.

Enhanced Strength: Even in his physically withered state, Kōga is deceptively strong. With a single downward chop, he was able to snap his sword's blade in two.

Immense Spiritual Power: While not a noble by blood nor of considerable age, Kōga possessed great power even at his young age. Ginrei, the 6th Division captain at the time, states Kōga's power vastly surpasses his own. He displays a massive reiatsu, even after being sealed away for several centuries. His reiatsu is purple in color. His skill and control at using his spiritual energy allows him to perform a variety of abilities.


 * Illusions: By channeling his spiritual energy into his opponent, Kōga is able to distort the target's perception, effectively making a person's five senses unable to properly register what is happening. This power can be negated by one using their spiritual energy to close off their emotions before the hypnosis takes place.


 * Spiritual Threads: Kōga is able to form invisible spiritual threads like his Zanpakutō to bind his opponent in place. These threads are visible on reflective surfaces such as ice.

Zanpakutō

 * Kōga's Zanpakutō takes the form of a katana with a dull red sheath, purple hilt-wrapping and a guard in a shape similar to the Kidō Corps' emblem, consisting of a circle with a pentagon-like shape in the center and three points protruding from the edges evenly spaced apart. Muramasa is an illusion-type Zanpakutō.


 * Shikai: The release command is "Whisper" (囁け, sasayake). Muramasa shines with bright purple light to engulf a considerable radius, affecting any and all targets within it.
 * Shikai Special Ability: Muramasa can break into the inner world of targets, similar to how a tape-worm leeches off its target, and forces them to bend to the user's will. It can cause a Zanpakutō's spirit to abandon its master and attack them. The ability causes the Zanpakutō's of the wielders in the general vicinity to seemingly take control of the wielders sword arm causing him/her to unwillingly attack their comrades or turn the blade on themselves. The only way to combat against Muramasa's power is by understanding how the Zanpakutō works and sealing ones "heart" against it. Though in order to block the attack in that state one must have incredible reiatsu and concentration. In addition, one must be able to still fight in that state.


 * Muramasa himself has the ability to manifest in his true form and even can manifest himself at a distance from his master, able to hear his call from wherever he is. Upon doing so, Muramasa has the ability to cause other Zanpakutō to manifest by controlling and warping their true instincts to the point of removing their ties with their Shinigami. When this takes place, the Zanpakutō of other wielders glow with a purple light. Their Zanpakutō spirits then manifest, typically as tangible shadowy figures from weaker Shinigami and with greater individuality and power from stronger Shinigami.


 * Bankai: Not Yet Achieved.