Kōga Kuchiki (朽木 響河, Kuchiki Kōga) 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.

Kōga is a tall and lean well-built Shinigami with very strong green colored eyes, dark purple hair with a long braided pony-tail of rich pink 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 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.

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.