The canon verse for Koro-sensei - in both the manga and the anime. He is goofy, fun, and an enjoyable teacher. He adores his job, and will always priorities the lives and safety of his students over anything else. Of course, he’s also routing for them to actually pull through. What sort of teacher would he be, if he undermined their goals by selling them short? He believes, without a shadow of a doubt, that his students are talented and special - and he’d do just about anything to help bring out that side of them, no matter how deeply it is buried.
On that note? If you’re muse is looking for a new teacher/mascot/parental figure? He’s happy to adopt.
In a verse were the experimentation left him a little unable to control himself - From time to time, Koro-sensei’s temper will flare. Usually do to pain and an inability to keep himself entirely stable. In this verse, he priorities his students the same as ever. However, he is frequent in reminding them the importance of their mission.
( He’s more unstable in this verse. And? SPOILERS: Unlike in the main story line, he is unable to be saved or left alone - he really will explode. So? Unfortunately, he needs to be killed. )
( Before the experiments that turned him into the worlds best teacher - Koro-sensei wasn’t the nicest guy in the world. Instead? He was a famous assassin. Known as the “God of Death,” he had never been known to fail. He is powerful, fast, and skilled. And? What is more so, he is good at blending in with society and sneaking out of places - Of course, all of the emotions he displays are fake.But? What does an assassin need real emotions for, anyways? )
Warning: This will likely be a bit of a darker verse than the others available for him.
In this verse, Koro-sensei will be training rookie assassin’s - This can be muses from other series, or the assassination classroom kids in a different verse, or whatever. But? Regardless, he’ll be training them the same way he trained the “Faux” God of Death. Teaching them, harshly, how to become professional assassins.