Academy Graduate at age 5, Chuunin at 6, Jounin at 13, Hatake Kakashi, the Copy Ninja of Konoha, is a legend among his own village, and well known outside it as well. Possessing an Uchiha-clan sharingan eye, he's made his nickname by stealing and copying nearly 1000 jutsu from the ninja he's battled. He trained under the man who became Yondaime, and was an elite ninja in the ANBU squad for many years. Finally pressed into service as a teacher to a genin team, he failed every team given to him for lack of teamwork - until team seven. Uzumaki Naruto, Uchiha Sasuke and Haruno Sakura became the first students to pass his test, and later, some of the most enterprising and powerful genin in Konoha.
Team seven completed many missions together, and though he tried to keep the two boys from becoming enemies, Sasuke ignored his advice and finally left Konoha, becoming a missing nin, and an apprentice to the Sannin Orochimaru. Going against the Hokage's command and putting an important mission on hold, all Kakashi could do was track Naruto down from the Valley of the End and carry him back to Konoha for treatment after he tried to bring Sasuke back on his own.
In the nearly three years since, Naruto and Sakura have never given up on finding Sasuke and bringing him back. While they both studied under the other two Sannin, Kakashi worked on his own skills, especially his sharingan use, developing his own version of the mangekyou jutsu. While recovering from the exhaustion his new jutsu caused, his team finally tracked Sasuke down, met him again, and discovered his strength had grown amazingly - too fast for normal means. Now it's up to Kakashi to somehow train Naruto quickly enough to surpass Sasuke, so they can bring him back - before Orochimaru takes him over completely.