Obito was one of Kakashi's former teammates. He is the one who gave Kakashi the sharingan. He is actually the identy of the masked man named Tobi of the Akatsuki
Obito was a young Ninja of Konoha and member of the Uchiha clan. He graduated from the academy at the age of nine, becoming a member of Team Minato. The team was led by Minato and consisted of Kakashi, Rin and himself. While the team could work with each other Kakashi and Obito would never get along. Kakashi followed the rules to the letter and was considered a genius of Konoha while Obito disagreed with several rules and hadn't learn how to activate his Sharingan yet.
At the age of eleven Obito made the rank of Chunin. But Kakashi had become a Chunin at the age of six and then at the age of thirteen he became a Jonin. He felt weak because Obito would always compare himself with Kakashi, creating a small rivalry. It was here where we are first introduced to the team in one Kakashi flashback during the Naruto series.
Team Minato had been given a mission to destroy the to stop enemies from entering Konoha. This would aid greatly in the Third Great Shinobi War effort. This was the same day that Kakashi was given the rank of Jonin and each member of the team had all brought him a gift, first minato gave him a special kunai and then Rin gave him a medical pack especially made for him. Kakashi placed his hand out to Obito who looked shocked and told Kakashi that he had got him nothing. Obito was jealous of Kakashi success in the ninja world and never really understood the young ninja. Kakashi replied that it was for the best as Obito would probably have got him something useless anyway.
As they left it was noted that Kakashi was now in charge of the mission and when they where faced with a group of ninja Kakashi lead the charge. Then team soon encountered a lone scout. Kakashi attempted his new attack, Chidori, even against Minato's orders. Meanwhile Obito was paralyzed with fear and began to cry in battle. Minato was able to finish the battle, disappointed at both students.
That night as they camped Obito spoke to Minato and asked why Kakashi was the way he is. Minato went on to tell the story of how Kakashi's father, Sakumo Hatake, had deiced to save his comrades and not complete his mission that lead to the death of many Konoha ninja’s. The village blamed him for those losses and he eventually blamed himself as well. Very few thought that he was in fact a hero for going back to save his friends. Later Sakumo took his own life and ever since then Kakashi has followed the rules to the letter. Obito now had a better understanding of Kakashi and the next day he began to follow Kakashi orders without much hassle. they were at the point of the mission when Minato would have to separate from the rest of the team.
But disaster struck when Rin was taken by two Iwgakure Jonin. Obito wanted to rescue his teammate while Kakashi said that the mission came first. Kakashi ironically found himself in the same situation as his father had been in the past and did not want to make the mistakes that his father had. Obito disagreed and left Kakashi not before telling him that what his father did in the past made him a hero not a disgrace. Obito was confronted by Taiseki, one of the Iwgakure ninja. He was soon over powered, just then a flash of silver filled the air and using his father’s sword Kakashi entered the battle just in time to save Obito. As the battle raged on Taiseki managed to strike at Kakashi left eye because Kakashi had used his own body to block an attack meant for Obito. Terror took over as he watched the blood escape his friends eye and wanting to fight back past the fear his Sharingan awakened, using his newly activated Sharingan Obito and Kakashi defeated Taiseki. They headed into the cave where Rin was.
It was here that Kakashi's life would change forever. Obito and Kakashi fought against the last ninja to save Rin. As the battle progressed, Obito and Kakashi worked as an unbeatable team and freed Rin but the victory was short lived as the final ninja caused a cave in. Obito dived in time to save Kakashi from a falling rock which landed on him instead. As Obito lay dying he gave Kakashi a final gift for becoming a Jonin: his left eye. Rin, using her medical techniques, implanted the eye into Kakashi.
Using his new gift Kakashi used the Chidori to break free of the cave in and killed the Ninja who had caused Obito's death. Just then enemy reinforcements arrived and Kakashi fought to protect Rin, telling her that he would not let her die because Obito loved her. Just as the battle was about to be won by the enemy, Minato Namikaze appeared and saved the two students and finished the mission with their help.
After that, Obito's name was carved in a memorial of all those Hidden Leaf ninja who died in action. Kakashi regularly visits the area to pay his respects to his friend and hero, Obito.
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