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.

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 Otogakure, where she served as a jailer at one of his bases. Unlike the rest of the team, Karin was indeed loyal to Orochimaru, going as far as to refuse to release Orochimaru's prisoners even after his defeat.

Personality
Karin has a fluctuating personality; she will at times appear tough and in control, and at other times will appear annoying and highly flirtatious. This annoys both Suigetsu and Sasuke, but she attacks Suigetsu, whenever he voices his opinion. She, like many girls in Konoha before her, has a huge infatuation with Sasuke Uchiha. Her infatuation, however, is an obsession 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 drug Suigetsu and Jugo to rape Sasuke's unconscious body (although her plans were ruined upon sensing the Konoha shinobi trailing them). Later in the manga, she reveals that she has kept one of Sasuke's sweat-drenched shirts. Karin also implies that a team consisting of just her and Sasuke would be much better.

Hunt for Uchiha
Sasuke Uchiha recruits  her as a member of Snake. While Sasuke admits she may be troublesome sometimes, he also notes that her special abilities make her useful. 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, 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 Suigetsu, Jugo, and Kisame as Sasuke went ahead to battle Itachi.

Hunt for the Eight-Tails
Team Snake, now renamed Hawk, later begins working with Akatsuki. They are sent to Kumogakure to capture the Eight-Tailed Beast's Jinchūriki. When they found him, Karin sat on the side lines and watched the others fight Kirābī. When Sasuke is mortally wounded fighting Kirābī, 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.

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 tried to revive Sasuke again. She was stopped by Jugo, who claimed she didn't have enough chakra to save Sasuke from such a great wound; so he asked her to help him heal Sasuke. When Kirābī fully transformed into the eight-tailed beast, Sasuke used Amaterasu to quell it. Some of the flames spread to Karin's cloak and Sasuke uses his Mangekyo Sharingan to put them out. Karin is carried to safety by Jugo.

While the team is recovering, Karin is shown to have now changed her look by putting her hair in a ponytail. She 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, the Mind's Eye of the Kagura, 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. It should also be noted that she had other bite marks on her arms, chest and neck.

Trivia

 * Karin's name is made up of the kanji for "fragrance" or "perfume" and "phosphorus".
 * Karin's infatuation for Sasuke leads her to have a number of similarities with Part I Sakura Haruno. Each also tend to beat up their other teammate (Suigetsu/Naruto) when angry.
 * Kishimoto has stated that the "thing" Karin did to Sasuke will be revealed in future chapters.