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.

(Source: BleachWiki)