Yachiru
Kusajishi

Yachiru Kusajishi is the lieutenant of the 11th Division and the small pink-haired child who rides on Kenpachi Zaraki's shoulder through every fight he picks. Kenpachi found her as an infant in the Kusajishi district, the worst part of the Rukongai, unafraid of the blood he was covered in, and named her after the only person he had ever respected.

She is also President of the Shinigami Women's Association, which she runs the way she does everything else, by deciding what happens and then doing it. She gives every captain a nickname they cannot escape, turns up at Yamamoto's tea ceremonies and Byakuya's calligraphy club purely for the food, and vanishes once the cakes are gone. Her sense of direction is famously catastrophic, which is why Kenpachi is permanently lost.

None of that is padding around a joke. Yachiru moves faster than most seated officers can track and is strong enough to carry Kenpachi on her back out of a fight he could not walk away from. She never reveals a Shikai on screen. During the Wandenreich invasion she takes on Sternritter without hesitation and treats the whole thing as a game she is winning, which is roughly how her captain approaches it too. The two of them work because neither has ever been afraid of anything.