Akaza/Synopsis

Human Life as Hakuji
Hakuji was born and raised in the slums to a sickly father he deeply loved. In order to pay for medicine, Hakuji pickpocketed from people in town, but was caught three times. The magistrate beat him and marked him with tattoos, threatening to cut his arms off the next time he was caught and calling him a "Demon Child". When Hakuji returned home after his third beating, a villager informed him that his father, having heard of his arrest, hung himself. In the letter he left, he stated he wanted Hakuji to live a full life and didn't want to take medicine if it was earned by dishonest means.

With his criminal tattoos and no home to return to, Hakuji was eventually banished from Edo. Devastated by the loss of his father and enraged at this society they lived in where he couldn't afford his father's medicine, he picked fights with people and beat them up. One day, after nearly killing seven adult men in a village, he was approached by Keizo, the owner of a dojo in the village. Annoyed by the man's cheerful demeanor, Hakuji challenged him to a fight, but was defeated with ease.

Keizo took him in so he could nurse Koyuki, Keizo's sickly daughter. Her mother, her previous caretaker, drowned herself due to stress and Keizo had to work handyman jobs to keep them afloat, so Hakuji was tasked with tending to her. Already accustomed to tending to his father, he had little trouble tending to her, though he found it awkward whenever she burst into tears. One of those times, unbeknownst to him, he had indirectly motivated her by telling her that "there will be a next year" to see the fireworks.

Later on he discovered Koyuki, whom was forced and left outside by the heir of the kenjutsu dojo next door. Thus, an angered Keizou and Hakuji fought the dojo next door. After Hakuji defeated nine people, he made them promise to never get involved with the Soryuu dojo and Koyuki ever again. The heir of the rival dojo got angry and lost his composure, so he swung an actual sword instead of a wooden one down Hakuji. Hakuji punched the sword from the side as it was coming down (called 'Bell Crusher'), and it broke. The dojo next door was moved by the beauty of this technique, accepted their loss, and apologized for the rudeness of the heir. Then they stopped harassing the Soryuu dojo.

Two years later, she was in good health and could stand and do chores. He continued training as Keizo's pupil of the Soryu style, and the dojo began to grow. One day, Keizo offered Hakuji to take over the dojo&mdash;and take Koyuki as his bride, to Hakuji's surprise and embarrassment. He agreed, silently vowing to protect both of them, even at the cost of his own life. At a festival, Koyuki told him of her past, asking if he was really okay with the proposal, to which he reaffirmed his vows to her, promising to protect her for the rest of his life.

The heir of the rival dojo heard about Hakuji's marriage with Koyuki. So he gathered some angered disciples and fought Hakuji, but they lost. Tragically, this victory was short-lived. He took a short trip to visit his father's grave and inform him of his marriage, but quickly realized something was amiss upon his return. A student from the dojo informed him that their rivaling kenjutsu dojo, which had always sought Keizo's land for themselves and prevented him from getting any students until Hakuji's arrival, had poisoned the well from where Koyuki and Keizo drank water from. The two died shortly after drinking that water while Hakuji had been away.

Enraged, Hakuji killed all sixty-seven members of the rivaling dojo with his bare hands, pulverizing them to the point where their corpses were no longer recognizable. This incident caught Muzan Kibutsuji's attention and, though he was disappointed to find it had been a human rather than a Demon who'd done such a gruesome act, he decided to make Hakuji one of the Twelve Kizuki. Hakuji, with nothing left to protect and having lost his will to live, accepts and becomes the demon Akaza.

Demon Life as "Akaza"
After becoming a Demon, he lost his memories of the time he became a human, with only one single desire to become stronger despite not knowing the reason why he wanted to become strong. Despite this, as noted by Doma, he refused to ever eat or harm women. Due to his strength, Muzan only dispatched him for special missions, such as searching for the Blue Spider Lily.

During his time as a Demon, Akaza noted that he had met and battled several Hashira of the Demon Slayer Corps during his hundreds of years of life. During these meeting, Akaza had invited every single one of them to become a Demon due to Akaza liking the natural strength, genius and skill the Hashira had possessed, wishing to continue battling against them forever. However Akaza was turned down by every single one of them due to them wanting to stay as humans, causing him much confusion as he could not understand their desire to stay as humans.

Although not exactly specified, it has been stated by Akaza that each of the Hashira that he battled did not use the Flame Breathing, showing that his battle with Kyojuro Rengoku was the first time he had come across a Flame Hashira.

Fifty years prior to his battle and death against Tanjiro and Giyu in the Infinity Castle, Akaza had come across and battled a powerful Water Hashira of noteworthy skill and power in the usage of the Breathing Style and successfully killed him and for the next half century did not come across anyone else of the same skill level until his duel against Giyu.

Mugen Train Arc
Akaza appears shortly after Tanjiro, Rengoku, Inosuke, Zenitsu and Nezuko have defeated Enmu and rescued the passengers. Noticing a downed Tanjiro on the ground, he immediately rushes forward to kill him, when Rengoku stops the attack and slices his arm in half. Quickly healing the wound and complimenting his blade, Akaza is then asked why he would attack an already injured person, to which he states he simply wanted to rid him quickly before talking to Rengoku. Deeming Rengoku as at the peak of his fighting spirit as a Hashira, he attempts to persuade him to become a demon. He states that as a human, he would eventually become old and die, whereas as a demon, he could continue to become more powerful. When Rengoku repeated refuses the offer and refutes his statements, an annoyed Akaza declares he would instead kill him, and activates Destructive Death: Compass Needle.

Akaza and Rengoku battle fiercely, with Tanjiro and Inosuke looking from a distance, both knowing the two are on a completely different level of power. Akaza is repeatedly cut but instantly regenerates his wounds, all while taunting Rengoku by saying all the Hashiras he had killed had never once taken his offer to change. The demon launches in the air and hits the Rengoku with Air Type, knocking him back temporarily, though the attack is quickly beaten. Knowing a battle at a distance will be difficult, the Flame Hashira speeds forward and tries to attack up close, though all his swings are deflected and dodged. Akaza again taunts his weakening human stamina compared to his demonic physiology and tries again to persuade Rengoku to change, which fails Rengoku repeats he will not become like him. They trade attacks again, with even Inosuke noting that if he tried to help Rengoku, he would be too slow and killed instantly. Akaza starts to make contact with his attacks, severely injuring the Hashira. All of Rengoku's sword blows are futile with the demon's rapid healing abilities and he starts to become desperate with multiple Flame Breathing attacks. Akaza suddenly backs away, allowing Rengoku to catch his breath, only for Tanjiro and Inosuke to see him bleeding profusely, in contrast to a completely fine and healthy Akaza.

Akaza, for a final time, taunts Rengoku for his numerous injuries, whereas his had already healed instantly become of being a demon and for fighting a losing battle as a human versing him. Rengoku's body suddenly becomes engulfed in fire, declaring he will fulfil his duty as a Hashira, to the demon's confusion and wonder. Rengoku decide he would try to injure him as much as possible, even if it means dying and rushes forward with Flame Breathing: Esoteric Art. Akaza is amazed at his opponent's fighting spirit, and decide he must become a demon so they can battle forever. Declaring he is the Flame Hashira, he uses the Ninth Form of his breathing, Rengoku, and speeds forward. Akaza responds with Destructive Death: Annihilation Type. They hit each other; with Rengoku slicing Akaza's arm, torso and almost through his neck, and Akaza driving his arm through Rengoku's solar plexus. Akaza desperately begs him to become a demon, knowing the wound he inflicted is fatal and amazed at his opponent's will to fight, declaring him one of the few strong chosen ones. Rengoku refuses once more, seeing a vision of his mother's promise to him to protect the weak, and drives his sword deeper through his neck, even catching his fist when Akaza tries to punch him with his free hand and shocking the demon. The sun begins to rise and Akaza attempts to flee but is held in place by Rengoku refusing to surrender. Tanjiro and Inosuke attempt to aid the Hashira, Inosuke using Fang One: Pierce to cut off Akaza's head. Akaza suddenly rips off his own arms from Rengoku's body and stomps the ground, creating a shockwave to knocks Inosuke away and giving him the chance to flee. Tanjiro frantically gives chase and hurls his Nichirin Blade at the Upper Moon, piercing his chest. Akaza hears Tanjiro's cries of calling him a coward for retreating, enraging him as he mutters to himself that he was fleeing from the sun, not him. Deciding the battle over and Rengoku will soon die, Akaza flees to safety in the forest, leaving Inosuke and Tanjiro with a near-death Rengoku.

Entertainment District Arc
Akaza appears before Muzan and reports to him that he could not find the Blue Spider Lily and that he took down one of the Hashira.