(This is for the fanfiction "Crossover Together". Info may change from canon information)
Korosensei is a cheerful, yet mischievous demon of higher intellect than most other teachers of Orora High, and tries to give proper education to even the most troublesome of students.
He does show some disagreement with Gakuho Asano's "ant system", and was shown to defy the system to give the E class a proper future in life, though his attempts weren't as forceful and upfront as Heine's involvement.
Despite his cheeky nature, he does hold a deep care and worry to his students, and humans in general that he knows closely, evident when he became overly distraught when Nagisa Shiota went missing after Pluton's attack on Amestris.
Korosensei's most iconic attribute is his speed, capable of moving at Mach-20, ten times faster than the fastest fighter jet. He is able to travel across the world in a matter of hours.
As a teacher, he is shown to have a considerable amount of intelligence on multiple subjects. This includes extensive knowledge of both the Underworld and Overworld.
Not much is known about Korosensei's youth. He once lived in the Underworld like many other demons, but traveled to the Surface World in order to "try new things", which eventually caught the attention of Military involvement, and as such placed a wanted status on him. It is unclear as to how he got to Orora High, or why he decided to teach 3-E, though this is seemingly under his own terms as Gakuho Asano was unaware of his involvement the year prior. He also gave a promise to not harm any student, teacher, or other staff member of Orora High in order to do so.
Once the first demons begin to come above to the Surface World, Korosensei took it upon himself to aid Team Harmony: He gave Yugure Kagayaki and Nagisa Shiota information about the Homunculus Oroborous Symbol, and he helped them in their fight against a pack of Houndour, each which he remained in the shadows due to keeping his identity a secret.
He made his first official appearance when Nagisa, Aladdin, and Blair went to the 3-E building to figure out the current state of 3-E, Korosensei with Karasuma in the middle of a training session. Eventually, Korosensei did catch them, saying how he already knew they were watching him, but didn't want to stop their observation and even agreed to answer their questions sometime afterward, calling Nagisa "cute" in a written message to Gakuho.
Once Karma Akabane entered his class, Korosensei welcomed him into their "3-E family", though ended up injured by Karma when his tentacle met with his hand, making Karma the first to injure him. He fell for more of Karma's tricks throughout the day, yet it failed to work, Korosensei considering himself a caregiver, and that every act he did was just another opportunity for him to do his part. When the day ended though, he tried his hardest to protect his students from an attacking Banette, and is overjoyed when he found Karma with his Miracle Sword. During Karma's first encounter with Envy, Korosensei arrived and managed to save Karma from getting killed by wrapping Envy up and demobilizing him, just before he carried Karma off to safety.
His role in season two is toned down to a side character, more often than not Korosensei appearing mostly as a comic relief, though he does get a main moment in saving Team Harmony, Victoria, and Seitenshi from an attack at the England Royal Ball; catching them in a hay wagon and placing fruit underneath the fallen balcony to trick Envy and Lust into thinking they were dead.
After his involvement in England, he returns to teaching 3-E, only to find Tataoka teaching his students instead of Karasuma. He was alright with it at first, just to find Tataoka brutally harming his students, yet he couldn't do anything due to his promise, and Tataoka technically being a teacher at 3-E now. This left Korosensei in a slump, explaining to Niji and Nabiku Hazukashi his frustration, though while he can't touch Tataoka, he couldn't leave them to get help either as he had to patch them up after every training session. Korosensei cheered Niji on during her fight with Tataoka, only to find out Karasuma was being transferred and Tataoka will be all they got, though he was just as glad as the rest of the students to see him go.
When Pluton began his invasion on Amestris, Korosensei had arrived to help kill him, helping Nagisa finish him off in a combined effort, only to lose Nagisa in the aftermath blast just moments after. He assumed Nagisa had died, and blamed himself for it.
Korosensei arrived to check on Team Harmony sometime after, and once he found out about Envy, he showed him that he included Envy as apart of the 3-E class, as a way to better educate him about living in the Surface World. After a particularly bad first day though, Korosensei explained to the class on who Envy was, and got them to promise to make him feel at home. Once the encounter with Tirek had ended, he quickly patched up Envy and healed him.
His involvement from there is a supporter to Team Harmony above much else.
He shows a great devotion and care towards many of the students of 3-E, willing to defy Gakuho Asano's "ant system" in order to give them a bright future, though his insecurities do make him slightly paranoid over their safety on occasion.
He has his respect towards Team Harmony, treating them as if they were apart of his class. This became even more so when some of them did become apart of 3-E, and once they ended up captured he became emotionally depressed. He quickly perked back though once they returned.
Thanks to his horrid treatment of his students, Korosensei developed a disliking to him, only growing more frustrated with him due to the loophole Tataoka got him in. He was very glad to see Tataoka go.
Korosensei had some knowledge of Heine prior to his arrival, and considers him his idol or master, constantly referring to him as "Heine-sama", much to Heine's dismay.
- Korosensei usually refers to himself as a Demon King, despite not truly ever being on in his life as pointed out by. This could be the result of his cheeky nature, and his natural power compared to Humans
- Despite this, he is very well known around the Underworld.