Yachiru Kusajishi was a character in the Bleach anime and manga series who served as the lieutenant of the 11th Division of the Gotei 13 and was a constant companion of Captain Kenpachi Zaraki. However, she disappeared under mysterious circumstances during the Thousand-Year Blood War arc. There are several theories about what happened to her, including:

• Gremmy killed her: Some fans believe that Gremmy killed Yachiru, as she disappeared right after defeating him.
• She is Kenpachis Zanpakuto spirit: According to some theories, Yachiru was actually the personified spirit of Kenpachis Zanpakuto. When Kenpachi fully activated his Bankai, she disappeared because she had merged with his Zanpakuto.
• She is permanently gone: In Can’t Fear Your Own World, it was stated that Yachiru is permanently gone since Kenpachi activated Bankai, but that Kenpachi still communicates with her from time to time. She also appears on Kenpachi’s shoulder while he uses Bankai, though it is unknown if that was only during the first time using Bankai or if it’s every time.

While there is no definitive answer to what happened to Yachiru, it is clear that her disappearance remains a mystery in the Bleach series.