>> There's barely any interesting development to the main character.

I thought the second and third book (The Dark Forest and Death's End) dragged on quite a bit, but the lack of character development is probably due to the fact that the author is not writing in the American tradition where a "character arc" is expected.

Think for instance of the two major works of European literature: the Illiad and the Odyssey. At no time is there any "character development" for Achilles, the main hero of the Illiad ("Sing to me, ô muse, the wrath of legendary Achilles"). He starts out as an egomaniacal, bloodthirsty asshole and he ends as an egomaniacal, bloodthirsty asshole. His best friend and lover dies because Achilles is an egomaniacal, bloodthirsty asshole; but does Achilles learn a lesson, say about violence, or egomania, or being an asshole? Does he "grow" and "develop" and "learn from his experience"? Hell no! He is Achilles, the demi-divine, nigh-indestructible death machine! He grabs his kit and goes out to take revenge for the death of his friend, every trait of his unhinged personality amplified ten-fold. He kills Hector and drags him behind his chariot, desecrating his body. He refuses to return the body to Hector's father, Priam, who comes to beg for it. He remains an accomplished asshole for the entire story. When he dies, everyone sighs a sigh of relief. Then Troy falls, by the cunning of Ulysses. The story is over. The hero hasn't grown, he hasn't learned, he hasn't been transformed by his experiences, there has been no character development- but there has been story development, aplenty.

Look also at Ulysses. I have no mean, righteous words for Ulysses. He uses his brain, although he has a momentary lapse of reason when he yells his name at Polyphemus, bragging that he is the one who blinded him, causing Poseidon to come after him to take revenge for his son. Ulysses and his men go through all kinds of epic, legendary encounters with mythic people and beasts, they even go down to Hades and speak to the shades of Achilles and other dead heroes, but there is again no discernible change to the character of Ulysses. He remains cunning, inventive, stoic, determined to return home, hunted by some gods, aided by others, a cursed leader who dooms his companions to destruction by the hubris of his deeds. But does he grow and develop? Not really and Homer doesn't spend much time discussing Ulysse's feelings, or his thoughts - he talks about his actions and through his actions we know what kind of man he is. When he finally gets home he murders all the suitors that beset his wife (and all the women of the house who so much as smiled at one of them), but is that act the result of something that happened in his journeys? Did he start as a peaceful, non-violent man, who learned to use violence in his long journey home? No. He is Ulysses, from start to end. And yet, again, look at the story! The character hasn't changed, but the story, it has moved forward, unstoppable. It is the story of an epic journey through a mythical land, but the man who started the journey is the same as the man who finished it.

I don't know when the idea of "character development became a staple literary cliché, but it is neither necessary to tell a good story, nor a universal characteristic of great stories. In my view, it's simply an easy trick to learn, a standard technique that everyone can master with some effort, either as a writer or a reader. But it should not be an expectation when reading a piece of literary work from an arbitrary place and time.