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Hatake Kakashi

Also known as: the Copy Ninja
Age: 26
Birthday: September 15th
Zodiac: Virgo
Rank: Special Jounin
Students: Uzumaki Naruto, Uchiha Sasuke, Haruno Sakura
Only Original Skill: Chidori
Hobbies: Reading adult books like "Come come Paradise" and being late.

Hatake Kakashi is the leader and teacher of Team 7, consisting of the series' primary characters. He is initially portrayed as a detached and apathetic figure, but as the series progresses, his loyalty to his friends and students becomes increasingly apparent.
His past is shrouded in mystery. His left eye is an implanted Sharingan eye which he used to copy over thousands of techniques, and earned him the nickname of Copy Ninja Kakashi. Team 7 is the 1st group of Genins that Kakashi passed, as it seemed that they have comprehended the importance of teamwork.
Kakashi's role in the series predominately consists of his time with Team 7, leading them on missions and training them to improve their ninja abilities. Later in Part II, Kakashi reforms Team 7, but because his students are now capable of taking care of themselves, Kakashi takes a more active role in the battles that go on during the series, particularly those with the criminal organization Akatsuki.

Umino Iruka

Age: 25
Birthday: May 26th
Zodiac: Gemini
Rank: Chuunin
Occupation: Pre-Genin at the Ninja Academy
Most Seen: Eating ramen with Naruto

Iruka Umino is an instructor at the Ninja Academy, characterized by the distinctive scar across his face that he has had since childhood. His parents were killed by the Nine-Tailed Demon Fox when he was younger, though he holds no hostility towards its container, Uzumaki Naruto. In fact, Iruka is one of the few people at the start of the series who values Naruto as a person, being able to relate with Naruto's methods of trying to gain attention. Because he, like Naruto, was forced to grow up without parents, and was socially isolated as a result, he can relate to Naruto in this regard as well and has assumed the role as Naruto's father-figure.
Once Naruto becomes a ninja Iruka starts to underestimate Naruto, and as such become overprotective of him, though he in time discovers that Naruto has all the makings to become a great ninja.

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For more info about the relationship between these two, take a look at the "about" page.

(sources: Wikipedia - 1 & 2, NarutoFever 1 & 2)