Kūhaku was born to a wealthy family on the outskirts of Keikokugakure. As a child he always dreamed of being a great ninja like the Keikage and thus trained extensively in the ninja arts. He was considered a prodigy in his own right as he easily learned everything that his instructors had given him and by the age of 5, he graduated from the academy and became a Genin. From there, Kūhaku continued to hone his skills and by the age of 7, he was promoted to Chūnin. This is also the same time when he discovered he possessed the Dark Release Kekkei Genkai, However one night when he returned home after a tough mission he heard screaming coming from inside his house and when he came inside to investigate, he found rouge ninja with his parents bodies on the floor, dead. Traumatised and enraged, Kūhaku unleashed a powerful blast of dark release chakra that killed the rouge ninja and blew up the whole house as a result. Kūhaku became a loner after that. His traumatic experience stopped him from resuming anymore missions and he refused to talk or be with anyone in particular. While he thought that his current attitude would never change, this changed after he met Shinji Hizumuro, as he told him that even though his parents might not be alive, they will never die as long as he remembers them and he reverted back to his old personality. He became a Jōnin years later and with his skills heavily improved on, he became one of the strongest Jōnin in the village
During his travels, Kūhaku encountered all kinds of strong ninja from various countries and battled them. As he experience grows, his dreams are for the world to coexist in peace and as time grows, his philosophy grows ever so stronger.
Kūhaku is a man of above average height with a slender yet muscular body. He has amber eyes and long blue hair that is waist-length that he kept either untied or in a ponytail. His usual attire consists of a kimono that was brown in the bottom half and white in the upper half, separated by a black line along with a black undershirt, outlined in red with the sleeves going up to his fingers and pants that turns red as they progress down. He wears a white strap across his waist, acting like a belt which is used to hold his sword and a blue rope holding a purple pendant. He wears black socks which leaves his toes exposed and sandals.
In his traveling attire he wears a stripped, crimson kimono with a black undershirt with a white outline. His sleeves go up to his wrists and he carries a purple sash on his right shoulder which is used to carry his sword. He wears grey pants with black socks that cover his knees and toes along with sandals. He is also seen carrying around a straw hat.
When he unleashes the full power of his Dark Release kekkei genkai, Kūhaku's hair turns black and grows slightly longer in length. The power emanating from him shreds any clothing off his torso and is replaced by a bluish-gray bandage-like carapace, made from dark release chakra which forms around his torso, right arm, and face, ending just beneath his eyes. Black, flame-like markings spread across his chest and wrap around his left arm, and the remains of tatted pants, which have darken from his chakra covering his lower body. In this form, Kūhaku's entire body constantly emits vast quantities of smoky, yet acidic black chakra, most of which is focused around his right arm.
In his childhood, Kūhaku had a kind and cheerful personality. He always had friendly sparing matches with his friends on a regular basis or playing ninja. He also visited them when they were sick to keep them company and always talked about how he wanted to be a great ninja. His instructors also said that he was cocky and hot headed.
Ever since the death of his parents, he became cold and withdrawn from others, preferring to keep to himself. While most of the time he remained emotionless, he had a deep longing for his parents to be alive and visited their graves every Sunday.
After hearing about the truth from Shinji Hikimuro, most of his original personality had come back. He also became more calm and serious, as he didn't want his teammates fooling around when there was a crisis at hand. Kūhuku possessed great control of his emotions, he didn't get worked up when someone was trying to make him and remained cool and collected during the vast majority of the battle. He also displayed a sense of honour, as he didn't kill anybody who have a high importance in a certain role e.g. Kage unless they were morally evil in someway. Kūhaku also didn't take any pleasure in killing anybody, preferring to incapacitate his victims unless he was left with no other choice. His dreams are for the world to have peace and equality throughout existence and he still holds on to that philosophy on a very high scale. He also displayed a lazy side of himself, as when he was to run as the replacement Keikage for 3 days he refused because all that paperwork would be to troublesome.