Obito Uchiha

was one of Kakashi Hatake's teammates in the manga exclusive "Kakashi Gaiden". He was a 13-year old Chunin in Team Minato with Kakashi Hatake, Rin, which was led by Minato Namikaze (the future Fourth Hokage). He's very similar to Naruto Uzumaki as despite being a member of the battle-adept Uchiha clan, his talents for the ninja arts were not particularly impressive. He was also shown to be goofy and desperate to prove he is a worthy ninja, mostly unrespected by peers and teachers alike. However, he is ready to put his life on the line for his teammates without hesitation. Obito carries the blood of an Uchiha and has the power of the Sharingan within him. He graduated from the Ninja Academy at the age of 9, and got promoted to Chunin at the age of 11.

Kakashi Gaiden arc
Obito was first seen desperately rushing to meet with his team, but arrived late and got scolded by Kakashi. Minato then states that as Kakashi is a Jonin, he will be in charge of Rin and Obito, while he reinforced other Konoha defense forces. Rin then reminded Obito that he needed a gift for Kakashi, but while he did not have one, Kakashi suggested that Obito would give him something worthless if he did. The group then received their mission to destroy a supply bridge to prevent a Rock village ninja invasion of the Land of Grass.

During the group's first encounter, Obito froze up in the heat of battle, prompting Minato to bring him to his senses. After Minato killed the enemy ninja and told Kakashi that teamwork was more important than the rules, Obito tried to rub it in, but was told that he needed to control himself in order to back up his words. Obito, again feeling inadequate, talked with Minato about Kakashi and learned his story.

The next day, Minato temporarily left the group for an urgent mission, thus leaving Kakashi, Rin, and Obito by themselves. Two Rock ninja tried to ambushed the team, causing Rin to be kidnapped. Kakashi argued that the mission came first, as diverting to save Rin would compromise it, but Obito argued that Rin was their friend, and they needed to save her. Obito then left, stating that while those who break the rules are trash, those who abandon their friends are worse than trash. Hearing that, Kakashi walks away from Obito to carry on the mission.

Shortly after Obito reached the location where Rin was held, a Rock Village ninja senses him and attacks Obito from behind, but Kakashi stepped in just at the nick of time to save him by slashing him with his sword. The ninja hid himself, and Kakashi lost his left eye saving Obito from an attack. Obito realized that the time had come for him to live up to his words, and activated his Sharingan for the first time. He saw and killed the ninja who attacked them both. After the two saved Rin, a third Rock Village Ninja caused a cave-in, and Obito was trapped under several rocks while saving Kakashi. The whole right side of his body was crushed, slowly killing him. When Kakashi went to his fallen teammate's side, in confirmation of their friendship and as a gift for Kakashi gaining Jonin rank (Obito was the only member of the team not to have given Kakashi a gift), Obito gave Kakashi his left eye, and the Sharingan with it. Along with the quote “I’m already going to die, but I can become your eye. And from now on I will see the future". After Kakashi defeated the ninja, the cave collapsed as enemy reinforcements arrived, and Obito died, realizing that he had finally become friends with Kakashi. Though he was unable to tell Rin how he felt about her.

Personality
Kakashi's trend of being late seems to have come from Obito. He was almost always late, and was carefree about rules, leading to a rough relationship between himself and Kakashi. While Kakashi believed that the rules were everything during his time with Obito, Obito believed that self-control and caring for one's friends was more important. Despite his carefree appearance, Obito often suffered from self-doubt, as while he boasted that he would surpass Kakashi once his Sharingan awakened, he later stated that he saw himself as a "washout" among the Uchiha’s. Obito had romantic feelings for Rin, but never was able to confess them to her. But in the Naruto Manga series, it shows that Rin understood Obito's love feelings for her even though he never confessed it.

Abilities
As a Chunin, Obito is proficient in all basic skills of ninja training. Also, as a member of the Uchiha Clan, he is proficient in Fire Release: Great Fireball Technique and basic Katon ninjutsu.

Upon activating his Sharingan, Obito showed himself capable of quickly adjusting to his newfound power and effectively used it to aid him in battle.

Appearance
Obito wore a ski mask-like pair of goggles with a see-through orange tint (the goggles have no color in the anime). He also wore dark blue pants and a jacket. His hair was spiky like Naruto's, but was a black color.

Trivia

 * Kakashi's quote "Those who violate the rules and fail to follow orders are lower than garbage. However... those who do not care for and support their fellows... are even lower than that!" is originally Obito's.
 * Both appearances by Obito in the anime have been still, he's never been animated. He appears first in Naruto episode 74 as a memory during the Third Hokage's speech about the will of fire, before being killed. Obito's goggles in this memory are colorless. His second appearance is in a photo, during the extra part of Shippuden episode 64. Now, he is in a photo and his goggles have the see-through orange tint they had in the only colored page of the manga he appears.
 * Kakashi's always late because he spends too much time at Obito's grave, honoring him after the Uchiha died.
 * When recommended that he should wake up earlier to honor Obito (thus getting more time to do important things) by Yugao Uzuki, Kakashi replies that if he comes by earlier, he will stay longer thinking about his regrets.
 * Many people believed Obito was posing as Tobi, since Tobi's mask showed only the Sharingan of the right eye, and Kakashi has the left. Also, The name Obito is almost an anagram for Madara's alias Tobi, with the subtraction of only one already used letter. However Tobi revealed himself to be Madara Uchiha, largely disproving this.
 * In a way, Obito is similar to Naruto; both were at first goofballs desperate to get attention, and to best their rival teamates (Kakashi and Sasuke, respectively). Both of them also had a crush on their female teammate (Rin and Sakura), who each prefered (Kakashi and Sasuke) instead.