|Affiliation||Demon Clan (spy -former-)|
|Abilities||Mach-20 speed, "Rage State"|
Korosensei is highly intelligent, yet mischievous in his behavior. While he does care for his students, he can act rather shrewd and annoying towards 'old friends', such as Greninja for example. Since he can move at Mach-20 speed, he doesn't really take any threats or insults seriously, often knowing he can just rush out of trouble once it shows itself without much danger to himself or his class.
Not much is known about his youth before he passed by earth.
As he visited the earth out of curiosity, he was confronted by Giratina's projection. After a discussion, Korosensei nearly refused until Giratina managed to convince him of a special reward for his effort, which he happily accepted after some quick thought. Korosensei joined The Duel of Yin and Yang, fighting for the Demon Clan as requested by Giratina. During the duel, he personally took battle with Greninja until Arceus and Giratine both lost, yet Korosensei was the only one who was left behind, not affected by earth customs or religions unlike the other fighters among the duel. He was peeved for no reward, but wasn't bugged him too much.
Sometime afterwards, he was contacted by Giratina again, still claiming on the deal, and gifted him with Mach-20 speed. However, he was blackmailed into the human study, otherwise he'll remove the ability from him. Not wanting his power removed, he agreed to the deal.
He had set his position at the Yunibāsaru Junior High School, as Class 3-E teacher. He went on through his days as a teacher, getting to know the students among the class, and taking various notes on how Humans work, like it was no different. While working as a teacher, he came across Greninja once again during gym, and assured Greninja that he wasn't planning on harming any student among the class. During the practice match with Greninja and Naruto, he came out and congratulated Naruto on his efforts (apparently a rare act from Greninja's perspective).
Korosensei eventually flew by the fight with Kaibutsu, and quickly won the fight. However, Korosensei lets Kaibutsu go, in favor that Kaibutsu won't attack anymore humans. After that, he took Nagisa and Karma, and returned to Tokyo.
He remained with the Class 3-E, teaching the class about various subjects, including astronomy, and taking notes for Giratina. He gave his notes to Giratina at one point, but was told to take notes on Nagisa Shiota. While doing so, Korosensei suddenly suffered from spring allergies, unable to teach his students. As a counter, Korosensei and Twilight both worked and created a substitute teacher, Ritsu. Korosensei stayed with Twilight while he was sick, and took some more notes on the humans. When he returned, he reintroduced Karma Akabane. However, he became the victim to many of Karma's pranks throughout the day, getting his tongue burned during the whole time. While talking to Giratina again, he was caught by the students, and quickly left upon Giratina's command. Korosensei tried to remain as a normal teacher afterwards.
As he was staying, Korosensei won a ne month trip over to Kenya, and he gets an idea to bring the students along for the ride. After some consideration and planning, Korosensei got who were allowed to go, and set off to Africa. While he was there, he was found by Pauli, who told him about the Destroyers from the Desert. Once hearing this, Korosensei took it upon himself to train Kimba, Weed, and Mel, for a rematch against them, which proved successful. He also helped in cleaning up the jungle after the Spider's defeat.
However, he was soon confronted by Giratina, who warned him dire consequences if he continues to help the others. This got Korosensei into a worried panic, and it only grew worse when he was dragged into helping fight Narcissus. When he saw his students struggle, Korosensei entered his "Rage" state, and betrayed Giratina by killing Narcissus himself, saving his students from harm. Later, Korosensei was found again by Giratina, and was simply told that they were no longer allies, thus making him an enemy of the Demon Clan.
Naturally, he is a very fast alien, moving extremely quick. But, his speed only increased once Giratina gifted him after the duel had taken place, able to move at Mach-20, the fastest thing in earth's history, making it like he's teleporting. This same speed also establishes to his maneuverability, something he uses often.
The "Rage State" is the name given to Korosensei's state of mind when his strength and anger are at its highest. This state makes him extremely aggressive, and very powerful against whomever triggers it, and its an ability unintentionally given to him by Giratina, due to the demonic enhance in speed.