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.
Hell King Bass is a cold, emotionless Evil Overlord who is not only behind every atrocity that happens in the first 2/3 of the manga, but also personally takes the field to give many of the main cast their tragic pasts. The worst thing he has done, though, is killing the greatest champion of humanity and one of his most passionate enemies, Lute, in an incredibly sadistic, spirit-shattering way, then turning Lute's body into a soulless puppet, forced to carry out his atrocities. And this was all done right before the eyes of Lute's mother and comrades. Once said body started breaking down, Bass intended to do the same to Lute's sister.
The Big Bad himself, Great Demon KingChestra, is the only villain in the whole manga who is always presented with dead seriousness (even Bass, despite having no funny traits or sense of humor by himself, serves as the butt of a few jokes), invoking nothing but terror (in good guys) and awe (in his minions). In the backstory, he seduced one of the kindest women in the world, Pandora, and pretended to be her loving husband for years, just so she will open the box for him, where his magical power and most of his mazoku minions were sealed, intending to kill and devour her once she outlived her usefulness He also had children with her, just to make them live traumatic lives and eventually devoured them, after they suffer enough and develop strong enough magic to serve as a powerup, restoring his full abilities. The worst aspect of this is that it is implied that, unlike the rest of the mazoku, almost all of whom are firmly stuck in Lack of Empathy and Evil Cannot Comprehend Good territory, Chestra actually felt love for Pandora, and still decided to proceed with his plan.
Cult Classic: Its Japanese fan base is already pretty small, but its American one is downright tiny, only enjoying a brief moment of popularity at the very end of the nineties that died out by the mid-two thousands.
Ensemble Darkhorse: Sizer came second after Hamel in the popularity pole while Raiel came in third. Lute, who up to that point had absolutely no characterization or backstory besides being Bass' puppet came in sixth place.
Evil Is Sexy: The manga clearly subscribes to this trope - Sizer's normal outfit is very modest, while "Dark Sizer" is Stripperific.
The most overtly sexual character is the Axe Crazy Vocal.
Ho Yay: Done very intentionally in the manga with Hamel and Raiel - the other characters flat out accuse them of being gay a few times. And let's not even get into Olin harassing Hamel. And then there's that scene when Clarinet is introduced, and the heavily implied crush Clari may have had towards either Lute or Horn (or both? It's never truly clarified) for most of his life.
Watanabe hangs so many damned lampshades in the manga that it's a wonder no one's mistaken it for some kind of deranged chandelier. He draws roses in the background during the yaoi parodies, for crying out loud.
Vocal acts no less touchy feely with his male adversaries as he does with his female ones.
Examples of Ho Yay above at no point go beyond gags, though. In Schelkunchik, on the other hand, you cannot help but suspect this, when Schel and Great fall into the same roles that Flute and Hamel played in the original series.
Iron Woobie: The entire Team Hamel and some of the supporting cast. After all the horror and torment piled on them, it's amazing that they aren't suicidal wrecks with the temporary exception of Sizer, anyway.
Magnificent Bastard: Arguably Chestra. Definitely Vocal. The sheer evil he puts Orgel through...
Narm: A lot of people consider the anime just as hilarious as the manga, only in a different way.
No Problem with Licensed Games: The video game is as fondly remembered as the manga, with its only major criticism being the short length removing ninety percent of the cast entirely.
Trom is a much lighter example. While he's not hated by the fanbase by any means, he is the least popular member of the main group by far and ranked considerably lower on the popularity poles as a result. In response to this, he was Put on a Bus for a large chunk of the story and is completely Adapted Out from the OVA.
Anything involving Lute (in the manga). He's tortured to death protecting his mother, Flute and Sforzando— having his eyes cut, his throat and legs stabbed, and his arms crushed. He is then possessed by Hell King Bass, and abused as his puppet for fifteen years... dying again upon his release.
Sizer's backstory. Kidnapped by and raised among the Mazoku since she was a baby, with Ocarina as her only friend. Forced to destroy countries since the age of six. Being lied to that her mother abandoned her in favor of her twin brother, then finally meeting her mother... who is sealed inside crystal. Spending the next 10+ years of her life massacring humans and rising through the Mazoku ranks, pretending to be on their side while waiting for her chance to strike back.
The Woobie: The entire main cast, more or less. All of them have lost at least one loved one, been betrayed, or experience fates far, far worse than death. Standout examples include Sizer, Tron and Lute.
The King of Sforzando! His wife, children and subjects all have no idea who he is, completely ignore him when he sacrifices himself and continue to ignore him whenever he tries to encourage them from beyond the grave.
Jerkass Woobie: Hamel and Cornet. Hamel is a massive asshole but is so broken inside you can look past it, while Cornet is an absolute Harpy but gets so severely punished by karma that you can't help but feel sorry for her.