Suou Pavlichenko. One side of the fandom loves her for being a Badass Adorable who undergoes strong Character Development, offers a fresh breath of air from Hei, and isn't quite as tension-breakingly overpowered. Others are not so kind to her, believing that she does not have a very distinctive personality and is often used as a vehicle for lolicon subtext and fanservice, which takes screentime away from focusing on established major characters like Yin, Hei, or Kirihara, and find her out-of-place for being a Magical Girl in a gritty seinen series. A few would even go as far as to blame her singlehandedly for the failings of Season 2.
Hei in Season 2, especially if you haven't seen the OVAs that explain his sudden personality change. Some fans find him to be a Jerkass Woobie with completely understandable reasons for his shift in behavior, and that it sets him up for a much more impressive He's Back moment. The rest just find him completely insufferable and the setup for his return to form utterly contrived.
GaidenOVA: Claude, real name Shichi/Xi-Qi, is the Big Bad who stands out from the normally amoral superpowered Contractors by his outright hatred of normal humans. Claude torments his enemies before killing them by using his powers to masquerade as one of their loved ones and seeks to turn Yin into the Person of Mass Destruction Izanami. Claude is even callous enough to turn her into Izanami when she is in a base of Contractors, knowing that this would mean that her powers would kill everyone on the base, showing he cares nothing for his own kind even while trying to make a world where they rule over regular humans.
Flower of Darkness manga, by Tensai Okamura:
Harvest serves as an Evil Counterpart to Hei, replacing Hei's respect for humanity with contempt, considering them to be weak and pathetic. Harvest possesses the titular "Flower of Darkness", which can be given to a person to grant them powers, but sadistically forces the recipient to kill their best friend in return for power, also giving the best friend a Flower to entertain himself with the struggle. To prove his superiority to Hei, Harvest distributes Flowers to all of humanity in an effort to prove to himself that there is no such thing as morality, then pettily tries to destroy the whole planet when his plans are foiled.
The girls' academy volleyball coach, Daisuke Mioka, is a handsome, charming man who takes advantage of his young students by seducing them and secretly making videos of the sex to sell on the internet. When one student finds out, Daisuke tries to win back her trust by having a group of his friends rape her, before sweeping in to act as the hero who saves her. Daisuke also promises his friends that he will be "a little late" so that they can begin violating her before he arrives.
Fanon Discontinuity: The finale of Season 2, for being needlessly depressing and confusing to the point of the utterly nonsensical and for accomplishing nothing in the long run. Some extend this to the entirety of the second season due to its huge change in tone and perceived inferiority.
Fan-Preferred Couple: Hei/Yin. The art for the series makes them look like an Official Couple but the series portrays the exact nature of their relationship as ambiguous. Yin developed feelings for Hei in the first season, the manga provided them with more Ship Tease, and the OVA ramped this Up to Eleven. However, they both have trouble expressing their emotions properly and Hei is a Master of the Mixed Message so his protectiveness towards Yin could be his Big Brother Instinct acting out. The official website states that Hei "feels strongly" about Yin and various staff members for the anime think of it as romance. There has been no official statement made on it though.
Foe Yay: Misaki and Hei, ever so much. In the second season, Mina jokes that she's saving her virginity for him.
Kiko's very existence serves as a comedy relief element for the series. Other than Mao's nervous sweat-beads, she's the only other character who's frequently prone to over-exaggerated anime emotions, such as Wingding Eyes whenever an opportunity to make money comes around, and Expressive Hair.
One great Kiko moment was when she and a friend were complaining about Adaptation Decay of some unnamed series.
And, after Mao wakes up in Amber's headquarters, he gives a long, dramatic monologue about how contractors always do the logical thing. When he get caught by the others for trying to non-chalantly walk out of the room, he tries to logically respond with a "Meow".
Most of the OVA. It's basically an omake parodying the main series, particularly mocking Hei's Chick Magnet status and the fact that a White Mask of Doom and a change of clothes really shouldn't be that effective of a disguise. Highlights include slightly loopy fangirl Mayu crushing on him due to his "sexy collarbone" and writing him into her yaoi fanfic (which Yin reads), and what is possibly the absolute silliest Reset Button in the history of fiction.
Harsher in Hindsight: For Troy Baker, who said in a Funimation commentary that at the time he played November 11, he had recently chose to quit smoking. However he also took it in stride and joked about it.
Episodes 11 and 12. Hei and Nick became close very quickly, spent a whole night watching the stars together (and made plans to go watch them again afterward), and Hei looked really sad and heartbroken after Nick "betrayed" him.
Female example: Buckets aimed at Misaki. She doesn't reciprocate much, though. In all fairness, pretty much every character has at least a little attraction for her, not just the women.
Havoc: Promise me one thing, though. If... If I revert back to my former self, kill me right away.
Jerkass Woobie: Huangwas a cop until his best friend was horribly killed... by the woman Huang loved, who he discovered had just been using him to get to said best friend. He was left with no choice but to join the Syndicate or have his memories wiped, which he feels would be like killing his friend all over again. This resulted in Huang being a bigottedjerkass.
Amber is an elusive, mysterious woman who betrayed MI-6 to work for The Syndicate, eventually leading the deadliest squad of Contractors in Heaven's War. Realizing the Syndicate's true intentions and using her powers to see and rewind the future countless times, Amber manipulated the Syndicate and her own side to seal off Heaven's Gate, which caused a massive disaster that rendered most of South America uninhabitable. Returning in modern day Japan, Amber orchestrates a series of bombings while kidnapping the scientist behind the Saturn Ring project, manipulating her former lover Hei and the world's intelligence agencies to place Hei close to Hell's Gate so he can seal it as well and prevent the Saturn Ring project from killing all the world's Contractors. After settling things with Hei, Amber sacrifices herself to rewind time and give him a chance to choose a third option, later revealing she had foreseen that possibility and left guidance in the new world Hei had made.
November 11, alias Jack Simon, is a suave Contractor and one of MI-6's best agents. First seen recovering the inactive former war criminal Havoc, skillfully reversing a double cross and eliminating everyone after interrogation, November later kills Havoc before she can reawaken to her Contractor state, showing himself the equal of Hei himself with a plan to nullify his electric currents. Later learning of the Syndicate's true reach and schemes, November defects to Evening Primrose and leaves his close ally Misaki Kirihara with clues she needs to dismantle the scheme, sacrificing his life to kill his former superior Decade in a choice his partner April describes as a rational decision and end that any Contractor would be proud of.
Hei will die. But his soul will choose to go back in time, the time when he was happiest, and he will be at season 1 first episode. Suou who loves Hei wants to be with him too and wants to be in a time when Hei'd love her, but since he already loves Yin she'll go back in time before Hei is born, and become his mother.note This copypasta jokingly places Hei, Suou, and Yin in the roles of three primary characters in the Shadow Hearts series. Said explanation is a note because it's also the ending to the second game.
Daisuke posting web porn of his conquests whilst they remain ignorant of it. With badly blocked out faces. Then hiring a gang to rape Azusa AND Kyouko, with Kyouko as an unwitting accomplice, just so he can "rescue Azusa" or so she can't tell on him.
If you somehow still harbored any sympathy for Genma, the throwaway comment in the last episode that he was the one who killed Youko and the implications of having raped her prior or even worse; after her death pushes him straight into total bastard territory.
Sequelitis: The second season is generally seen as inferior to the first. Various reasons cited include: Suou being seen as a much less compelling protagonist, Hei's personality change making him completely unlikeable, Yin's removal from the main cast taking away a great deal of pathos, a gratuitous uptick in sexual themes including Squicky lolicon fanservice, the mysterious atmosphere giving way to confusion for confusion's sake, and an overall decline in animation quality.
In season 2, Genma invokes the trope in hopes that it can apply to him ("Hey, having an unfortunate past can score me some Moe points!"). note That'd probably work better if Hei didn't have the Dark and Troubled Past thing cornered and Amber, Suo, and Yin weren't covering the moe.
Woolseyism: The first two episodes make more sense and fit the tone of the rest of the series quite a bit better in the dub; in particular, they changed Jean's completely-inconsistent-with-everything-else comments about "installing a personality" to something like "Contractors have feelings too" and replaced Mao's inexplicable line about Hei's coat only being bulletproof when he wears it with an excellent Deadpan Snarker moment along the lines of "Hei doesn't just wear that coat as a fashion statement."