Hatake Kakashi, or "Copy Ninja Kakashi" is Sasuke's, Sakura's and Naruto's sensei. Usually discrete by nature about everything including himself, it seems that he wants to keep who he really is a secret to others and he always keeps his lower face hiddenby a mask. Kakashi also has a reputation of being a severe and demanding teacher, having returned all of his previous students before Naruto and his teammates to the academy. He said that all the others were brainless kids that didn't know what teamwork is and that they only obeyed to what they were told to do, teamwork seems to be very important to Kakashi. But, even though he seems to be cold sometimes, Kakashi loves his student and care for them enough to protect them at all costs, even his life, and he demonstrated that with Orochimaru and Zabuza.
Kakashi is quick thinking and seems to always be one step ahead of everyone and almost always knows how to act accordingly, even though he's always late to every appointment he has, important, or not! Moreover, Kakashi always has an ethically questionable book on him from the "Come Come Paradise" trilogy, and he even reads it in his first combat with his students which, of course insulted Naruto and got him into some "trouble"!
Like a lot of characters in Naruto, Kakashi's past is a tragic one, dirtied with the death of many friends and relatives. First, his father, Hatake Sakumo (AKA: White Fang), was disgraced after he abandoned a mission to save his friends, but because of that, Konoha suffered great losses and the village and his companions blamed the White Fang for that, thus, making him weak of mind and body because of the guilt he felt for his failure, and ultimately leading him to suicide. And for that reason, Kakashi became very strict about rules and regulations until 13 years ago...
It is known that Kakashi became a jounin 13 years ago (from where the manga currently is), at age 16. At the time he was training under Konoha's future fourth Hokage, Yellow Flash. He was in a three man team with Uchiha Obito, whom he never got along very well because Obito never obeyed rules and regulations, and Rin. At the time, there was great turmoil between the great countries and Konoha was deeply involved in what was to be called: The Third Secret World War. All four ninja were involved in a sabotage mission during the war and went to Rock Country. During the mission, the future fourth had to separate from Kakashi and the rest, so Kakashi was given charge of Obito and Rin.
Unfortunatly, Rin was captured by enemy ninjas and Obito went to rescue her without Kakashi who was still stuck up with his rules and regulations (he wanted to resume the mission without her). Obito had found Rin, but was about to be killed when Kakahi, who finally understood that teamwork and companionship was more important than rules, stepped in and injured the rock ninja. After some battle Kakashi injured his left eye and they were both about to be killed when he was helped by Obito who had his Sharingan awakened and killed the camouflaged enemy. The enemy's companion that was cornered used a Doton jutsu to make some rocks fall on Kakashi's team. Rin got out of the way but Kakashi was hit by a big rock on his blind side and fell to the ground, in an ultimate act of courage, Obito pushed Kakashi out of the way, therefore, saving him. Half crushed under the rocks, slowly dying, Obito's last gift to Kakashi was his Sharingan in his left eye, that was transplanted by Rin into Kakashi, making Kakashi what he is today, always being late and privileging teamwork to carry on Obito's legacy.
Afterwards Kakashi became an ANBU team member for a number of years, his only known mission was a rescue mission to save Snow Country's princess. But, judging from what we have seen in that part, we can deduce that Kakashi has been a normal jounin only for a few years.
Kakashi's fighting technique is complex and complete. First of all, Kakashi is quick thinking and has a developed sense of smell, thus making him very receptive to dangers surrounding him. He's also an excellent scout because of his great sense of smell and his ability to summon dogs. Kakashi, more than Sasuke, has learned how to use his Sharingan to the best of it's capacity. He knows the genjutsu that are related to the Sharingan and he can easily confuse powerful enemies like Zabuza with it. Moreover, Kakashi's favorite use of the Sharingan is it's copying ability. It is said that Kakashi has copied more than a thousand techniques with it, making Kakashi's arrays of techniques an exceptionally wide one.
Finally, Kakashi can even foresee the enemies moves with his left eye, making him even more shrewd. Aside from his Sharingan, Kakashi devised a formidable and powerful jutsu named Chidori/Raikiri. With this jutsu, Kakshi can virtually pierce everything from rock to flesh. Adding to this great array of ninjutsu, Kakashi is a formidable taijutsu user, and proves to be a strong enemy to even taijutsu users like Gai and can easily hold his own in close combat, like he does against Zabuza. The son of the once respected White Fang can wisely use genjutsu to his advantage in combat, and he can use a lot of them, making him a dangerous adversary even when the enemy is in hiding or keeping a distance.