Not having a name to go by for as long as he could remember, he eventually chose to take his surname from the district he once resided in, Zaraki, which was District 80 of the North Alley of Rukongai. District 80 was the worst, most lawless region of Rukongai, full of thieves and murderers (whereas area #1 was the most orderly and law-abiding). As a young boy living in the streets of the Rukongai, Zaraki, coming upon the dead body of a Shinigami, took his Asauchi, later imprinting his soul upon it to make his own Zanpakutō.[22] During that time, he eventually became proficient in swordsmanship, and killed numerous opponents. Sometime during this, he, coming upon Yachiru Unohana, fought with her. The fight was the first time he felt fear, and subsequently the first fight he ever enjoyed, having been bored slashing anything else. He subconsciously sealed his power to allow them to fight longer and consequently lost to her. After their fight, he began to admire her, and wanted to fight like her.
Sometime later, as a young man, Zaraki, coming upon a young girl, named her Yachiru, in remembrance of the only person he ever admired, after she showed no fear of his sword. Yachiru came from District 79, Kusajishi, where her parents were murdered, seemingly not too long before Kenpachi arrived. On that day, he took on a name: Kenpachi, the title bestowed upon the strongest Shinigami in every generation, the one who has slain the most enemies and won the most battle.
Later, he had an encounter with Ikkaku Madarame and Yumichika Ayasegawa before they entered Seireitei. Yachiru warned Ikkaku messing with Kenpachi while he was in a good mood was a bad idea, though Ikkaku did not take the warning seriously. When Kenpachi and Ikkaku meet face to face, the latter immediately became alarmed by the former's Reiatsu, but was happy to finally have a real challenge. Kenpachi attacked with enough force to create a crater in the ground under Ikkaku, which somewhat alarmed Yumichika. Due to Kenpachi's strength, Ikkaku was forced on the defensive, holding off Kenpachi's relentless strikes. As the two continued to fight, Ikkaku tried to overwhelm Kenpachi, though he failed in doing so, only succeeding in increasing his own wounds while his opponent remaining untouched by his blade. While Kenpachi mentioned he was having fun, he decided to finish Ikkaku with one quick strike, resulting in his loss of the fight.
Ikkaku became mad Kenpachi would just walk away without finishing him off. Kenpachi, plainly telling him he had no interest in weaklings who could not fight, noted he did not have any obligation to finish Ikkaku off. Ikkaku, thinking he was toying with him, demanded Kenpachi kill him. Kenpachi, losing his patience, grabbed hold of Ikkaku and asked why he was screaming for death if he loved to fight so much. Kenpachi explained to him a philosophy to follow for people like them: "Don't admit defeat and seek death. Die first, then admit defeat. When you lose, but don't die, it just means you were lucky. At those times, think only about survival. Survive, and think only about killing the guy who failed to kill you". Kenpachi, smiling at him, stated it was not like he went easy on him; he was just lucky to escape death. He said Ikkaku should live, come back, and try to kill him again. As Kenpachi went to leave, Ikkaku, asking him his name, is told he is Kenpachi from Zaraki.
Sometime later, Kenpachi killed Kenpachi Kiganjō, the captain of the 11th Division, in a single blow in front of over two hundred members of the Division. As per the 3rd Captain Qualification Trial, Kenpachi became the new captain of that division. By killing the 11th Division captain, he became the official and uncontested holder of the title, Kenpachi, as well as becoming the 11th person to hold the title
The following day, during Kenpachi's first addressing of his division, he was greeted by Ikkaku and Yumichika, who had already become Shinigami and joined the division. Ikkaku told Kenpachi he had come back just as he suggested, causing Kenpachi to smile. Later, they were present at the battle of the division against a caterpillar Hollow. While initially the division lost, Ikkaku showed up to fight it, but was stopped by Kenpachi, who decided to kill it himself. After the Hollow was dispatched, more came, prompting Ikkaku and Yumichika to join in the fight. Though at first he was against it, Kenpachi was persuaded upon realizing the two had similar fighting traits, and, as such, would like to fight by his side. After this, Kenpachi allowed them to participate in his fights.
Shortly after joining the Gotei 13, Kenpachi was forced by Genryūsai Shigekuni Yamamoto to learn Kendō because he had not learned any formal swordsmanship training, having bypassed those normally necessary qualifications due to the way he attained his position. Central 46 ordered Yamamoto to stop the training after one day, as they became concerned that if Zaraki realized his full potential and rebelled, he could become an unstoppable force. Kenpachi hated using Kendō because his battles would end too quickly with it, so he only used it if his own life was in danger