I Quit Being the Demon King from the Start, Went Home to Raise My Little Sister, and Became a Rider

This is the story of a modern-day siscon who sleeps himself to death, then wakes up and somehow ends up in another world, having achieved the "Starting with Nothing but a Pair of Underwear" accomplishment. He scrapes by there for a hundred years, achieves the "Demon King" accomplishment, and finally finds a way back to his sister. Reunited with her, he returns to being an incurable siscon and starts stirring up trouble with her.

But... watching his sister transform in the distance and slaughter monsters, the siscon man falls into deep thought:

A skintight suit? With adorable frills? And floating cannons?

Whoa, is this a tech-based Magical Girl transformation!?

Without a word, he pulls a belt from his fourth-dimensional butt and slaps it around his waist.

He bends low, sets his feet firmly, and raises both hands:

"Trans-form!"

This novel and its characters are purely fictional. Any resemblance to actual people or events is purely coincidental. Do not imitate.