I’m the Hero, and My Travel Companion Is the Bratty Demon Lord

[Japanese-style light novel, Western fantasy] [Single female lead] The hero Lane thought he had slain the Demon King, and was about to let loose at the victory banquet when the System told him the real Demon King was still alive! What’s more, a war between the gods would break out three years from now, and the entire world would cease to exist! To survive the calamity, he would have to rely on the Demon King’s power! But when Lane finally found the so-called real Demon King… Why was the Demon King an impish girl? And why didn’t she have even a trace of magic power? Worst of all… Why was this impish Demon King looking at me so strangely?!