Bleach planted one of its most surprising mysteries in plain view for decades. Lieutenant Yachiru Kusajishi bounced through Soul Society's corridors, wielding her own Zanpakuto and commanding admiration as Kenpachi Zaraki's most trusted subordinate. Her vivacious conduct masked a truth that would redefine everything readers understood about Zanpakuto-wielder relationships.
Yachiru Kusajishi is Kenpachi Zaraki's Zanpakuto spirit, Nozarashi, in physical form. She embodied as his Bankai's detached aspect, gaining free existence when Kenpachi named her. This essence explains her mysterious disappearance during the Thousand-Year Blood War Arc and Kenpachi's unusual Bankai achievement.
Kenpachi’s first encounter with Yachiru took place in Kusajishi, the rough 79th District of Rukongai. After one of his brutal battles, a nameless infant came to him without fear. The child touched his blood-soaked, unnamed sword with complete trust. This moment, initially materializing as a simple character plot in Bleach, actually illustrated the first contact between Kenpachi and his Zanpakuto spirit.

The benevolent captain decided to adopt the mysterious child rather than abandon her. He named her Yachiru after the only person he had ever respected. This act of naming was crucial, as it endowed the Zanpakuto spirit with the ability to maintain physical form and develop Shinigami-like powers.
Names hold immense significance throughout Bleach's universe. They serve as bonds that anchor beings to existence and define their identities. When Kenpachi gave the nameless child the name Yachiru, he unknowingly empowered his own Zanpakuto spirit to exist independently.
This concept aligns perfectly with established Bleach lore. Characters like Ichibei Hyosube demonstrate how names can be manipulated to control existence itself. Yachiru gaining abilities through receiving a name follows the same fundamental principles that govern the series' spiritual mechanics.
Foreshadowing throughout the manifestation theory in Bleach

From her very first moments in Bleach, Kubo subtly dropped clues hinting at Yachiru’s real identity. In Chapter 114, she explicitly states she never had a name or parents before meeting Kenpachi. Despite remembering specific details like cloud formations from that day, she couldn't recall having any previous identity.
The same chapter delivers Kenpachi pondering the pain his Zanpakuto might feel from having no one know its name. These seemingly innocent dialogues take on deep meaning when viewed through the lens of later disclosures about Nozarashi's true nature.
Powerful souls in Bleach can split themselves into separate entities when necessary. Examples include Stark and Lilynette, who separated due to loneliness, and Szayelaporro, who split to shed violent tendencies. Yachiru's existence follows this established pattern.
It’s widely believed that Nozarashi took the form of Yachiru to forge an emotional bond with Kenpachi. Rather than appearing as an adult woman (her true form), she chose to manifest as a child, knowing Zaraki's compassionate nature wouldn't allow him to abandon an infant in the wilderness.
Bankai achievement and the mysterious disappearance

During the Thousand-Year Blood War Arc, Yachiru mysteriously vanished during Kenpachi's battle with Gremmy Thoumeaux. All that remained were her Shinigami uniform and the badge marking her rank as lieutenant. This disappearance overlapped with Kenpachi finally calling out his Zanpakuto's true name for the first time. When Kenpachi accessed Nozarashi's power directly, Yachiru returned to her rightful place as his sword spirit.
Kenpachi's seemingly instant Bankai attainment during his fight with Gerard Valkyrie puzzled many Bleach fans. However, the technicalities become clear when assuming Yachiru's role. Traditional Bankai requires externalizing the Zanpakuto spirit in the physical world.
Since Yachiru had existed outside the sword for years, Kenpachi had unknowingly been training for Bankai throughout their partnership. When she returned to Nozarashi, he could immediately access his Bankai because he had already fulfilled the primary requirement through their long relationship.
All in all, Yachiru’s existence as Nozarashi's bodily representation in Bleach justifies every anomaly surrounding her character. This twist recasts what emerged to be a simple mentor-student relationship into something far more deep. She was a Zanpakuto spirit desperately desiring connection with her wielder via the only means available to her.