Yachiru Kusajishi is the Lieutenant of the 11th Division in the Gotei 13, under Captain Kenpachi Zaraki, as well as the President of the Shinigami Women's Association.
Eleventh Division Lieutenant Yachiru Kusajishi is a very small young girl with pink hair. She wears the normal Shinigami robes, and thus far we have only seen her with the Lieutenant insignia on her arm. She appears to be the youngest as well as the smallest, among the captains and lieutenants.
Yachiru is child-like in most aspects, as 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, even though 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 like she gave a man the name "Maki-Maki" and later when he reminds her of that name she finds it funny. The anime suggests that Kenpachi simply walked in and killed the former 11th Division Captain, thus becoming the current one, so it stands to reason she became a Shinigami at the same time.
In an interview, Tite Kubo mentioned that he plans on revealing Yachiru's namesake.
Yachiru's favorite free-time activity is eating sweets, though she will eat pretty much anything. She attends Head Captain Yamamoto's tea sessions, Captain Unohana's ikebana lessons, and also Captain Kuchiki's Calligraphy Club, but only to eat up the snacks and cakes served during these occasions and then disappear within moments.
It is also known that Yachiru's specialty is infiltration, which she uses mostly for mischievous deeds. During the time when 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 doing various, mostly childish pranks and other "tasks" to please her.
Yachiru seems to show a little-girl crush on Byakuya Kuchiki. She refers to him by affectionate nicknames like "Bya-kun" and "Byakushi," invites him to come eat with her (which she never asks anyone else) and at once point is seen offering him her drool-covered lollipop. Byakuya seems to find her crush annoying and mostly ignores her, although he apparently keeps buns on hand to jam into her mouth if she gets too assertive. In a recent omake, she has been seen dressed up in a cat costume, catching the koi in Byakuya's pond. While visiting Ukitake and "helping" him finish his food, Rukia mentions that there are no more koi in the pond, which Byakuya is upset about.
She 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 had killed several other men with his unnamed Zanpakutō, and was completely unfazed by the violence she had just witnessed or the blood on the 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 ever admired. Since that day she has spent much of her time clinging to his back for transport.