Yugito Nii

is a kunoichi from Kumogakure. When she was two years old, she became the Jinchūriki of the Two-Tailed Monster Cat.

Hidan and Kakuzu arc
She is first seen running through the sewers from some unknown persons, later revealed to be Hidan and Kakuzu. When she enters a large room, where all the sewers seems to meet, she finally stops. After some bickering between the two Akatsuki members, she reveals that she intentionally led them there and activates an explosive tag on the tunnel behind them, sealing them off. Knowing that they are from Akatsuki, she swears upon her name that she cannot let them go, and she transforms into a miniature version of the Two-Tails. However, after a 30 minute unseen battle, she is taken down by Hidan and brought to the Akatsuki lair by Zetsu for extraction, thus killing her in result. It's possible that Zetsu devoured her body afterwards, as he usually eats anyone's body who has any involvement with the Akatsuki or its objectives.

Jinchūriki Tranformations
The only part of her abilities ever seen in the series was the ability to turn into a version of the Two-Tails, giving her an upgrade in her abilities, such as strength and an ability to breathe fire. The way she transforms is very similar to Naruto, being slowly covered in a dense blue fire chakra that takes the form of her Tailed Beast. It would seem that although she lets go of her control over the Two-Tails, she still has some control over the transformation and how much of its chakra that she can use.

Trivia

 * Yugito and Kirābī are both Jinchūriki of Kumogakure.
 * "Nii" means "second place" in Japanese.
 * Although Yugito did not have a lot of screentime, she has became very popular in the fandom.
 * On the back cover of Naruto book number 35, instead of Yugito her name is Yukito.