WN: Ankoku Kishi Monogatari
This particular WN is one where the protagonist is summoned... by the demon lord to save him from the hero! The protagonist, Kuroki, becomes a Knight of Darkness and faces off with his trauma - the hero in question - and begins a story of endless misunderstandings and plot twists.
That is, in fact, the peculiar flavor of this story. Over 90% of Ankoku Kishi Monogatari's characters are self-absorbed pricks on some level, with the gods being the worst examples of them all. The protagonist, Kuroki, and the demon king Modes are the most humane characters in a world full of narcissistic men and women. On one level, this is hilarious as it leads to an endless series of misunderstandings, turn and turnabout. On another level, it is exhausting, because everyone misunderstands everyone else. There are no Eureka moments, there are only a bunch of self-absorbed powerful deities, heroes, and monsters who all fail to understand each other's motivations on any level.
Kuroki is no exception to the misunderstandings... but in his case, he is just really pessimistic about how people see him and his own abilities (despite actually being one of the top most powerful beings in existence almost from the beginning and working his ass off to get more powerful).
The human side of things is Hellenistic, with independent city-states working loosely together on matters they have in common and fighting one another when they aren't. Humans are much, much weaker on an individual level than even goblins or orcs, (incidentally the biggest hole in the setting is the fact that humans are still alive at all) and so their trade routes are constantly endangered, and food production is war.
The demon side is feudalistic, with Modes at the top as the king, and his vassals all taking care of their own lands. The demons are not pure evil, but a lot of the Nargol races are cannibalistic toward humans while at the same time seeing them as aesthetically pleasing (in a sexual manner). This brings me to the second major hole in the setting... the fact that every species sees the human form as aesthetically pleasing (even ones like Arachne, Empusa, and centaurs). I'm pretty sure the writer did this on purpose, but it drives me a bit crazy that a giant scorpion finds human women more attractive than his arachne wife.
Overall, while this can be a fun read, it can also be infuriating at times.
Edit: Adding more details about the characters... The protagonist Kuroki and one of the hero's party, Shirone, are childhood friends/osananajimi, and Reiji - the hero - is a rival who traumatized Kuroki and was the cause of him becoming obsessed with martial arts. Shirone is a dreamer who loves heroes but is too self-absorbed to realize she is constantly hurting Kuroki's feelings with her idolization of Reiji. Reiji is a handsome but animalistic guy who goes out of his way to save girls (he seems to have an instinctive grasp of where to find women who need him) and thinks nothing of guys. Reiji is one of the top two narcissists in the story, whereas Shirone is just a blockhead.
Alrena, the 'main heroine' of the story, is one of the world's goddesses and the one who summoned Reiji and his party of girls. She is a self-absorbed woman who dislikes most men (including Reiji), but particularly dislikes Modes for his hideousness and the fact that he used her hair to create his wife, Mona. She is really good at faking compassion, but the truth is she cares little for most people beyond how they are useful to her. This kind of personality is actually rather typical of the more powerful characters on the human and Elios pantheon's side of things.