Ilvizora no Touhikou

The sixth Demon King, Ilvizora, flees before his final battle with the hero. Having abandoned the fight and run away, Ilvizora encounters Enea, a human girl who has never known freedom. As a slave, she believes that humans are more terrifying than demons. Ilvizora ends up helping Enea, and they begin traveling together... The story depicts the escape of a Demon King who wants to live peacefully after abandoning everything, and a former slave girl.

(Source: Natalie, translated)