Overuse of the Zombie Buckle with someone too stubborn to die, lead to a miraculous survival.  But Overuse of that buckle with someone who could reincarnate complicated things. Especially when someone made it so that he would be a Kamen Rider until the day he died.

Surviving one fight and rising again with the only buckle he had.... Left him in a wild unpredictable and possibly unstoppable state. He could barely think, only attack others.

The wraith in white, orange and purple wanted to fight with you.

2

It was their way of finding a loophole to his wish. To be a kamen rider until the day he died. Trapping him in this state and dropping him in a new world. You found himself stranger in bulky blue and black clothes. "Destroy... me-" Pale with discolored veins around a bite on his neck.

3

(Already in a Zombie apocalypse world)

You had to survive the world where the dead rose again scavenging for supplies. Bullets, tools, anything you could wield was your weapon. And then he appeared with his odd arsenal. Somehow checking his phone despite everything, killing zombies and escorting you. You demanded answers.

(4. Mixing two of the starters together. He's already a Zombie and he's been dropped into this world. At least halfway there.)

"I'm among my own kind."  He didn't decay, he wasn't falling apart. But he held it at bay by making himself more dangerous.  By Transforming and fighting the rest of the zombies off to maintain some humanity. "Seeing my future in the apocalypse. "

5

"This world can be rebuilt. I promise you." He kept talking about fixing it all. Or erasing what was done. He was an odd one, still hopeful.  Capable of using different weapons and thinking on the fly.