Yachiru is child-like in most aspects - she is small, cheerful, energetic, and carefree most of the time. She is often seen latched onto the back of her captain, Kenpachi Zaraki ("Ken-chan" as she calls him, "Kenny" in the English Dub and manga), just over his left shoulder, and is somewhat territorial about this position. She is surprisingly intimidating at times, especially when annoyed. Like Kenpachi, she has no sense of direction, but even so, he relies on her for directions. Yachiru resents being criticized in any way and will often attack the perpetrator. Further reflecting her child-like nature, Yachiru likes to give people nicknames. For example, she gave Makizō Aramaki the name "Maki-Maki" and later when he reminds her of that name, she finds it funny. She later renames him "mini-mustache". She also calls Ikkaku Madarame "Baldy".
It is also known that Yachiru's specialty is infiltration, which she uses mostly for mischievous deeds. During the time Kenpachi takes his usual afternoon nap, she goes to play around Seireitei. She appears to have dug secret tunnels through the Kuchiki house, as she was seen searching for Ichigo Kurosaki ("Icchi" as she calls him), to show them to him. The tunnels are shown again when Lieutenants Isane Kotetsu and Nemu Kurotsuchi try to take a photograph of Byakuya Kuchiki.
Yachiru is the president of the Shinigami Women's Association, and holds its funds as such, spending it mostly on toys and candies. She stores her "treasures" in the hideout on the Kuchiki grounds mentioned above. It seems that she uses the whole Association to order the other members around, who are made to do various, mostly childish pranks and other "tasks" to please her.
Yachiru first met Kenpachi in one of the worst districts of Rukongai, the 79th district of the North Alley, Kusajishi, as a nameless child. She crawled over to him after he killed several other men with his unnamed Zanpakutō, and was completely unfazed by the violence she had just witnessed, or the blood on his sword. Even as Kenpachi warned her that the sword was dangerous, she reached out and touched it, giggling as her hand was covered in the blood of dead men. He adopted her after that, saving her from certain death in Rukongai. He named her "Yachiru" in memory of the only person he had ever admired. Her surname Kusajishi was given to her because she was from that district. Since that day, she has spent much of her time clinging to his back for transport.
Shortly after Kenpachi killed the previous 11th Division captain and the pair joined the Gotei 13, Maki Ichinose informed Yachiru and Kenpachi of his intention to leave the Division. She asked him if he was really going to leave and if he hated Kenpachi. When he denied that he hated Kenpachi, she asked him why he was leaving. He then told the pair that the previous captain was the only person he could accept as his captain and, with Kenpachi's permission, he left.
The name and ability of Yachiru's Zanpakutō are both unknown. She carries it around with a loose cord, rather than at the sash of her uniform. Her Zanpakutō's scabbard is also unusual in that it has a pair of wheels for transport, which were installed by Ikkaku Madarame, who turns out to be handy with tools, after she continually pestered him to do it. The tsuba is shaped like a flower.