Demon Slayer: Reborn as a Lower Rank Demon, with Firefly as My Wife

Chang Jiang transmigrates into Demon Slayer and becomes Lower Rank Six. He doesn't want to work for Muzan, just find somewhere to slack off, but then he finds Firefly in the snow, on the verge of death from entropy loss syndrome.

To save her, Chang Jiang defies his demonic instincts and gives her the first bite.

Who knew Firefly would become a perfect demon and then shyly point to the snowdrift beside them? “Um... Brother Chang Jiang, could you help me dig out my mecha?”

From that moment on, the whole world of Demon Slayer goes off the rails.

They encounter a Hashira? Chang Jiang: “Don't attack! We're good citizens!” Firefly quietly puts on her Sam mecha, its engine roar shattering half a mountain.

They encounter Muzan? Chang Jiang: “Boss, I'd like to quit.” Firefly activates her “Complete Combustion” form, and green flames blast straight through the Infinity Castle.

He planned to use his knowledge of the future to save Kanae, Kyojuro Rengoku, and the others whose fates had left him heartbroken. But he discovers that when their powers combine, they can unexpectedly rewind time in the middle of a battle!

Chang Jiang: “So we can do more than kill demons. We can reload a save, too?”

Firefly, blushing: “As long as I'm with you, Brother Chang Jiang, I'll go anywhere...”