A filler character from the Zanpakutō Unknown Tales arc.

Zanpakuto: Muramasa

Kōga has piercing green eyes and shoulder length black hair and a long ponytail of red hair, with a short tufts of red hair on the left side of his head, and bangs that hung on the right side of his face to frame it. When he was a member of the Gotei 13 he wore a standard Shinigami Shihakushō, along with a long red scarf and a light lavender obi sash, with a beaded-necklace with a pendent. He also wore an intricate white headpiece called kenseikan on the left side of his head to symbolize his nobility that held his ponytail in place with red tufts of hair sticking out (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.

Kōga is 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 being betrayed and 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.

Several hundred years before the current time, Kōga was welcomed into the Kuchiki family as the son-in-law of the Ginrei Kuchiki. He soon gained much respect throughout Soul Society. He in his capacity as the 3rd Seat of the 6th Division participated in a battle between the loyalist and rebel factions of Soul Society winning a decisive victory for the loyalist side single-handedly. For his role Kōga was given much praise and accolades for Captain-Commander Yamamoto himself. Ginrei grew increasingly concerned about Kōga's overconfidence in his powers and made a point to address that he needed to to control himself. Kōga took this in stride but never could understand why Ginrei never acknowledged any of his achievements. He would increasingly speak to his Zanpakutō, Muramasa about this only increasing his overconfidence. As he engaged in battle after battle winning each one with ease, for his efforts he is given a special position by the Captain-Commander, making him the captain of a special force directly under his authority that is totally dedicated to wiping out the remaining rebels. 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. They achieved this by having their own men attack Kōga knowing he would easily dispatch them and then charging him with the murder and having him arrested. Ginrei arrived to see what happened but upon one of the fallen soldiers admission that Kōga sacrificed them for his own battle lust, he had no choice but allow Kōga's arrest until he can determine a way to clear him. Ginrei visited him in jail and detailed that his punishment had been handed down. He is for the next few hundred years to be suspended from his seat in the 6th Division and it was determined that his Zanpakutō was a threat to the peace, therefore its powers would be nullified. Ginrei noted to him that he didn't believe any of it but that the top brass had a problem with his Zanpakutō and its ability. He further explained that they weren't afraid of him but Muramasa getting out of control. Ginrei then left Kōga there to learn patience as he went to rectify the situation. Kōga was later freed by Muramasa who inspired him to take control of his life again.

That night Kōga killed the elites who framed him. Upon hearing of this Ginrei went in search of Kōga and found him in the outskirts of Soul Society. Ginrei confronted him about his actions, and Kōga explained that he killed the elites for their betrayal, hoping that deep down, they would regret their choice to frame him, but he had found them laughing. He noted that even after killing them he was still filled with so much anger for what they had done to him and the humiliation he had endured. Ginrei remarked that any chance at proving his innocence is gone and that his rashness now put him in a worse position and he will probably be executed. Kōga, upon hearing this, suffered a breakdown and accused Ginrei of being like all the others and being jealous of him. Kōga decided to stage a personal revolt against Soul Society and all his enemies. As he killed more people his instability grew. When Muramasa noticed that Kōga's heart has become turbulent, he tried to tell Kōga to forget the past and focus on the future, but Kōga became obsessed with the idea that everyone hated him and that he couldn't understand why. While fighting against a group a Shinigami, he attempted to release Muramasa and found there was no response. He tried again and Muramasa appeared. While he sat alone in a cave Muramasa appeared as asked him why he would kill innocent bystanders, to which Kōga remarked that he was teaching Soul Society a lesson for exiling him from their world. Soon he was 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. Eventually Genryūsai Shigekuni Yamamoto and Ginrei Kuchiki confronted Kōga, and after an exchange of words, they realized that they couldn't get through to him. They engaged in a minor battle and Kōga went to release Muramasa with no results. The two captains take advantage and move to seal Kōga away when they realized he lost total access to Muramasa's power.

Kōga's revolt was never recorded in the Soul Society's history.

(Source: BleachWiki)