Yachiru Kusajishi disappears after Kenpachi Zaraki learns the name of his zanpakuto. It is later implied that Yachiru was the manifestation of his zanpakuto spirit.
In the Bleach series, Yachiru Kusajishi is introduced as the lieutenant of the 11th Division under Captain Kenpachi Zaraki. Throughout the series, she is portrayed as a cheerful and mischievous young girl with a strong attachment to Kenpachi. However, her true nature is shrouded in mystery for much of the series.
As the story progresses, particularly during the Thousand-Year Blood War arc, Yachiru’s presence becomes more enigmatic. During a battle, Kenpachi finally hears the name of his zanpakuto, “Nozarashi,” and achieves a new level of power. Shortly after this event, Yachiru vanishes without explanation, leaving behind only her lieutenant’s badge.
The series does not explicitly state what happened to Yachiru, but it is heavily implied that she was not a separate individual but rather the personification of Kenpachi’s zanpakuto spirit. This is further suggested by the fact that she appears to help Kenpachi unlock his Bankai, a feat that typically requires deep communication with one’s zanpakuto spirit.
The ambiguity of Yachiru’s disappearance has led to various fan theories and discussions. Some believe that she returned to being a part of Kenpachi’s zanpakuto after her purpose—to help him communicate with his sword and achieve Bankai—was fulfilled. Others speculate that she still exists in some form, as zanpakuto spirits are not known to simply vanish after a Bankai is achieved. However, the manga does not provide a definitive answer, leaving Yachiru’s fate open to interpretation.