Sharingan

The Sharingan is a bloodline limit, or Kekkei Genkai, which appears in members of the Uchiha clan of Konohagakure. Some think that it was derived from the Byakugan many years ago. It allows the user to comprehend the attack of an opponent almost before the attacker knows what attack they are making. The Sharingan also allows the user to see the flow of chakra within a person, but to a lesser extent than the Byakugan.

When the Sharingan is first activated by the user, it has one to two tomoe around a central pupil. The weilder of the Sharingan, through continued use, evolves the Sharingan which involves gaining tomoe. The maximum number of tomoe in a conventional Sharingan is three. The only level up from there is the Mangekyo Sharingan, which is said to be gained by killing one's best friend. This bit of information must be taken with a grain of salt however, because Hatake Kakashi developed the Mangekyo Sharingan during the two-and-a-half year period of time when Naruto was being trained by Jiraiya. Three people are currently known to have the Sharingan in any form, they are Uchiha Sasuke, Uchiha Itachi, and Hatake Kakashi, who obtained his Sharingan from Uchiha Obito, one of his original teammates.