In a clearing in the woods stood a silver-haired man. It was impossible to tell his age because almost his entire face was covered, leaving only his right eye exposed. Still, he was probably quite young, because he held a book whose title was clearly visible-Icha Icha Paradise. The title alone made it obvious that this was not something children should read.
And this man, who looked quite respectable but was actually anything but, was none other than the Kakashi-sensei Naruto and the others had mentioned: Konoha's Elite Jonin, Kakashi Hatake, nicknamed the Copy Ninja!
As for why he was called the Copy Ninja, the reason lay in his covered left eye.
If Kakashi lifted his Forehead Protector and revealed his left eye, you would see a blood-red eye-the Sharingan, which only members of the Uchiha Clan could awaken. Its abilities included enhanced perception, copying Ninjutsu, and hypnosis. Those basic functions alone covered Taijutsu, Ninjutsu, and illusion techniques, the three main forms of ninja attack. As for Kekkei Genkai (basically an innate talent; either you were born with it or you weren't) and the like, those were cheats and did not count as conventional methods of attack. The Sharingan itself was a Kekkei Genkai.
Kakashi had used the Sharingan's ability to copy Ninjutsu to copy over a thousand techniques. And because he possessed chakra of all seven attributes, he could freely use techniques of every type. It was simply absurd.
Although the Sharingan was a Kekkei Genkai, it differed from other Kekkei Genkai in that it had a physical form. This also meant that as long as you found a member of the Uchiha Clan who had awakened the Sharingan, you could gouge out his eyes and transplant them into yourself. This was also how Kakashi had obtained his eye.