Karin

is a former subordinate of Orochimaru who assisted him in his experimentations and was left in charge of one of his bases while he was away. She has red eyes, matching hair color, and an unusual hairstyle: her hair is short and unkempt on one side and long and combed on the other a sign of a fluctuating personality; Karin will at times appear tough and in control, and at other times will appear flirtatious. Karin's name means is made up for the kanji meaning "fragrance" or "perfume" and "sulfur". It is also a homonym for the fruit "quince".

Background
Karin lived in a small village that was burned out during a war. She was the only survivor, completely unharmed. Orochimaru randomly found her after that and asked how she survived, and answered that she could tell huge groups of people were coming. Orochimaru then brought her to Oto, where she served him as a jailer at one of Orochimaru's bases. However, Sasuke had at some point noticed her at the time of his defection and had decided to use her in Team Snake long before he rebelled against Orochimaru. Unlike the rest of the team, Karin was indeed loyal to Orochimaru, going as far to refuse the release Orochimaru's prisoners even after his defeat. Suigetsu has mentioned (perhaps as an empty threat) that Karin did something to Sasuke in the past.

Personality
Karin usually appears as tough and prone to violent outburst, her usual victim being Suigetsu. However she, like many in Konoha before her, has a huge infatuation with Sasuke Uchiha, to the point of fanaticism. For example, when Team Snake was resting in a village to take care of the wounded Sasuke after his battle with Deidara, Karin planned to use a drug on Suigetsu and Jugo to put them to sleep so she could have her way with Sasuke, and planned to keep one of his battered, sweat-drenched shirts for herself.

Hunt for Uchiha
Sasuke Uchiha recruits her as a member of Snake. While Sasuke admits she may be troublesome, he also notes that her special abilities make her useful. She also seems to have a liking for Sasuke, implying to Sasuke that a team consisting of the two of them would be much better than one with Suigetsu Hozuki or Jugo. Once the team is completed, Sasuke sends "Snake" out in multiple directions to try and locate Itachi Uchiha. While on the search, Karin passes Sakura Haruno, though the two do not acknowledge each other. After Deidara's suicide explosion, the rest of the team regroups and takes Sasuke to an inn to recover. When Karin went out for supplies, including a drug that would allow her alone time with Sasuke, she sensed the 8-Man Squad approaching, and warned the team. It was her plan to use Sasuke's old clothing as a means to distract the Squad from catching them. When they finally got back on Itachi's trail, Karin stayed behind with Suigietsu, Jugo, and Kisame as Sasuke went a head to battle Itachi.

Karin, and the rest of "Snake" were present with Sasuke and Madara Uchiha when he renamed the team "Hawk" for their new goal; the utter destruction of Konoha.

Hunt for the Eight-Tails
Later, she and the rest of the team go to the Lightning Country to capture The Eight-Tailed Beast's Jinchūriki. When they found him, Karin sat on the side lines and watched the others fight Kirābī. Soon, after she started to mock Kirābī's rapping abilities as Sasuke began to fight him. Karin believed the fight was in the bag as Sasuke began the fight but she saw proven false as Kirābī easily mortally wounds Sasuke at the fight's begining.

Karin immediately rushed to pull the dying Sasuke out of the fight as Suigetsu and Jugo kept Kirābī busy. As she pulled the swords out of Sasuke's body she exposed her right arm, riddled with numerous bite marks. She then pleaded for Sasuke to bite her arm and as he did her chakra healed his wounds. Sasuke acknowledged that Karin had saved him as she doubled over in pain.

As Kirābī got bored and attempted to leave, Karin used her chakra sensing powers to locate him. Kirābī then used his demonic chakra to charge at her, Jugo, and Suigetsu but Jugo rocketed them away to safety. After Sasuke was yet again critically wounded by Kirābī, Karin revealed that her chest was also covered with bite marks and she attempted to save Sasuke. She was then stopped by Jugo, claiming she didn't have enough chakra to save Sasuke from such a great wound; so he asked her to help him heal Sasuke. She then watched in horror as Kirābī transformed into the Eight-Tails. Karin then helped Jugo move Sasuke as Suigetsu held the Eight-Tails off, but as he was defeated even Karin, who despised Suigetsu, felt sorry for the pain he was in.

It seems that when Sasuke used Amaterasu on the Eight-Tails, Karin almost got hit with one of the tentacles, but Sasuke used some sort of Chidori variant to cut the tentacle off. While Karin was trying to recover, the Amaterasu spread and hit her. Sasuke put out the flames with his Mangekyo Sharingan and had Jugo use his Cursed Seal to elongate his arm to grab Karin out from the water.

With the mission over, she was carried away.

The Wait for Pain
As her team rest up, Karin is shown to have now changed her look by putting her hair in a ponytail. She scolds Suigetsu about his anger towards getting his sword left behind. She then claims that Suigetsu should be lucky that they saved him at all. She then notices that something is wrong with Sasuke but as he brushes it off she detects an enemy outside their base which Sasuke and Jugo go to investigate.

Abilities
Karin's special ability allows her to track specific individuals through chakra, seen when she is able to discern that Jugo is not among a crowd of prisoners through the standard concentration pose for molding chakra. It also allows her to pick up chakra from long distances and notice changes in an individual's chakra. Kakashi Hatake surmises that Karin is a particularly skilled chakra-sensing ninja, as she is not only able to tell that his group is approaching, but can also sense how many of them are present and that one is a dog. Karin also has the ability to heal others by letting them bite her, and suck in her chakra, when Sasuke bit her arm (after being injured to the brink of death by the Eight-Tails), his wounds healed. It should also be noted that she had other bite marks on her arms, chest and neck, so most likely the marks of one of those bites are permanent.