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 "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.