Isaribi

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

Background
The test 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.

Sea Country 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. Naruto was able to make her come to her senses, causing her to realize that, were she to continue killing others to save herself, she would become a true monster. After his defeat, 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

 * Isaribi, although an anime-only character, gave Naruto his first kiss with a girl (although it was the kiss of life - CPR).
 * Unlike Amachi upon regression back to human form, Isaribi's ripped clothes are somehow restored intact when she returns to human form.