These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Compressed Adaptation: The manga changes a lot from several characters. Saya is often a Jerk Ass, Hagi is more sensitive, Kai is more mature early but acts like a pervert towards Saya, Riku gets more screentime, David gets less screentime, Solomon is more like Amshel, etc. The plot and pacing also differ greatly from the anime. These changes make it look as if they were aiming to attract more male readers due to the bigger focus on bloody action scenes and added Fanservice.
Complete Monster: Amshel Goldsmith. He and the original Joel's experiments turned Diva into a bloodthirsty monster that she is.He also has no qualms about killing anyone, including Diva's own chevaliers, if he feels they are getting in the way. In fact, it's revealed in the end that he only cares about Diva as a specimen that satisfies his intellectual curiosity.
Saya's Victory, rightfully placed amongst the Greatest Battle Music Of All Times.
Designated Hero: In the two manga series Saya comes across this. When she remembers her past she blames Hagi for abandoning even though she took down his arm. Saya's intention to kill everybody from her race are revealed earlier and Riku is simply told to deal with it. In both manga series, the climax has Saya brutally murdering an Anti-Villain.
Fridge Brilliance: Those who were curious enough to google the lyrics to Diva's aria (we don't blame you) found out that the words were nothing but gibberish. This perfectly illustrates Diva's background. Because she was only given the bare minimum to survive, she never had the chance to go to school (unlike her sister) and thus never learned human language until much later in her life. By then, she was already singing this song, which reflects her anger, frustration, and downright depression from the way the first Joel and Amshel treated her.
Growing the Beard: When Saya grows her hair out. Possibly not the most accurate timing as that is subjective but things did seem to get more interesting around the time she was reintroduced with her hairstyle changed.
Ho Yay: Nathan and James, Moses and Karman, and Karl and Solomon in the anime, Riku and Charles, and Hagi and Nishi in the prequel manga.
Moral Event Horizon: Diva crosses it when she rapes Riku so that she can bear his child, and then kills him.
Van Argiano and the American government cross it when they plan to turn people around the world into Chiropterans and then choosing who to send The Cavalry to, effectively controling the world.
James Ironside crosses it when he tricks Moses into hunting down Kai with the false promise that he can cure him and his friends of the Thorn.