Jason Kurenai is the best friend of Hajime Nagumo, he's been by his side since they were kids. Hell, some people mistook them as brothers with the way they act with each other. But in school things are different, Hajime is treated poorly by others, leaving Jason to protect him at best he can, he even threatened his classmates to leave him alone and not pick on him. Although, he ends up takin some of the abuse as well, he has some other friends, other than Hajime, to protect him. All in all, things were pretty boring for these two.
Then one day, Jason, along with the rest of his classmates and their teacher, was summoned to fantasy world, and was asked to help save it. During a dungeon raid, Jason and Hajime got betrayed as one of their classmates launched a spell directly at them, causing them to fall. Although Jason was fine, his friend Hajime ended up having his arm sliced off and nearly bled to death. They managed to escape, but neither of them has any idea of what to do now. Suddenly, he remembered why he fell in the first place, angered by the revelation that one of their classmates tried to kill them on purpose, and nearly succeeded, the two made a promise to find a way out and find a way home. Both doesn't care who stands in their way, if somebody dares to try and stop them, THEY'LL KILL THEM!
During their journey, they'll meets a vampire girl, a bunny girl, and a perverted dragon girl. These three will join them on the journey, and they won't be the only ones.
It is just a normal day for students at a certain high school, before things suddenly change. They get summoned into a fantasy world. They are treated like heroes and requested to save the human race from extinction.
However, there are some lies behind the entire incident, hide something even worse. As the students live their new way of life as heroes, there are those who slowly wonder what is going on.
As an incident take them to the deepest part and the most dangerous of the dungeon, they found power that rivals even the gods of the world. But in order to gain that power, they will have to do one thing...
...discard their own humanity.
(A/N: I replace Nagumo Hajime with different MC, and while there will be no harem, the pairing either one or two girls only. I do not own Kamen Rider franchise or Arifureta series. I follow both anime and manga setting because there are some part in the manga that the anime skipped or shortened)