the author run out of actual interesting content after the romance of the main pair. even if the romance was better handled in the manga then in the novel (IMO- since in the novel, he has sex with Risu on the first night after they met and he "defeat" her.) In the manga, they kind of build something before they go intimate but after this, in the novel, it goes downhill pretty fast. (If they follow the novel)
Basically that Demon that Fulio and Risu took with them (who's gender isn't defined ) grows a penis and
rapes the Mage girl daily just as an experiment to see what sex is !??? ...ok... and the Hero who is actually a fraud, when he took stuff from the treasures of the castle, he will unleash yet again another catastrophe, countless people die and guess what, he is forgiven and actually gets a harem... that's "comedy" for you
This story is decent until chapter 14, I guess, where Risu and Fulio consume their relationship, as the title of that chapter is "Farewell" and that's what you are supposed to do now. I really liked this series but after checking the novel,... what a disappointment. Yes the main couple has kids in the novel later on, and that's great but with all this that comes together, it's ruined