Isaribi

was a human test subject under the control of Amachi, who in turn used to work for Orochimaru.

Background
Isaribi was part of a test that was meant to give shinobi the ability to breathe and operate underwater. Isaribi was one of the first test subjects, and was not the final version. She was sent back to the Land of the Sea to spy on the ships leaving port. Due to her bandages, which were used to cover the permanently scaled parts of her body, the villagers attacked her and vandalized her property. They feared and hated her, as she was the only person to return from being "spirited away". Because of this, Isaribi is very cynical of anyone who tries to be friendly with her (she had much distrust for Naruto Uzumaki when they first met). She was very distant because of the hatred she experienced. Naruto described Isaribi and himself as being very similar, if not exactly the same.

Land of the Sea Arc
Isaribi was promised that she would be returned to human form if she complied with Amachi's orders. To that end, she attacked ships and killed a number of sailors under the disguise of the Kaima, a sea monster.

She saved Naruto, who had been assigned to investigate her attacks, from drowning at sea and nursed him back to health. Naruto quickly learned about how she was hated by the villagers when a group of children attacked her. When Naruto and his team discovered her role in the attacks, Naruto tried unsuccessfully to talk her out of it, telling her that if she continued, she really would become a monster. Isaribi refused, believing he had no idea of her suffering.

She later took part in another attack, only for Naruto's team to intervene, and ended up battling Naruto, who once again tried to convince her to stop her attacks. He later battled Amachi, intending to force him to cure Isaribi. During the battle, Isaribi learned that Naruto was Kurama's jinchūriki, and had endured the same suffering she had. With this revelation, Isaribi gave up on Amachi.

After his defeat, Amachi tried to get Isaribi to help him escape, but she refused, reiterating Naruto's warning to her. Amachi revealed that he had never had a cure for her condition, and that he had planned to dissect her once she was no longer useful. She returned with Naruto to Konoha after Amachi's defeat, hoping that Tsunade could cure her.

Trivia

 * Unlike Amachi upon regression back to human form, Isaribi's ripped clothes are somehow restored intact when she returns to human form.
 * Isaribi is one of the few characters in the series to be seen wearing tabi socks, and one of an even fewer amount of characters to be seen wearing them without sandals or any other type of footwear.