Berserk Button: Badmouthing Nicolas is one way to make Worick consider attacking an unarmed woman.
Beware of the Nice Ones: Right as Alex is about to turn an alley during her search for one of Worick's clients' missing cat, she hears the threats and beatings of a bunch of men to a homeless, half-starved-looking Twilight girl, and just as they begin talking about doing worse to her, Alex suddenly leaps out to smash the man saying so in the face with a steel pipe. Definitely awesome.
Comedic Sociopathy: Dr. Theo trying to stab Nicolas with a poisoned syringe for letting his hospital get damaged by the black Twilight man.
Cool Old Guy: Chad the police officer and Monroe the mafioso.
Worick and Nic are in their 30's so technically they are too.
Cool Old Lady: Big Mama Georgiana, the head of the Corsica brothel. She's even nice to Nic.
Creepy Child: Mikhail, who began the most recent Twilight Hunt that has led to the escalating violence and massacres against Twilights, has no sense of the height of what he's done besides "wanting [their] dog tag," and seems to have enjoyed slaughtering those he had already done so, possibly due to being raised and twisted into thinking that way, considering his young age.
Lamarck Was Right: Twilights are most often the children and descendants of Celebre users. Celebre was a body modification drug created during wartimes to enhance strength, speed and agility, all of which it did, at the cost of being mostly toxic in large or repeated dosages. Besides gaining physical strength similar to their parents, Twilights are most often affected by genetic, mental and physical deficiencies and are not particularly long-lived (hence the name "Twilight"). Because a Twilight must take Celebre to control their withdrawal symptoms, it's assumed that if a Twilight has children, they will also be Twilights.
Meaningful Name: Many, but the name of the city the Benriya lives in stands out—Ergastulum, derived from a term for the dark confines whereupon Roman slaves were chained up and/or punished by way of back-breaking work.
The Napoleon: While not a midget, Nicolas is shorter than most of the other men in the manga. Mentioning it doesn't send him into a violent rage but it does annoy him. He certainly doesn't like women taller than he is.
Doug, the kukri-wielding Twilight, turns out to be an adult with a body that hasn't aged properly since he was 12 (although his larynx seems to have developed). Either way, he's a total badass.
She Is Not My Girlfriend: Inverted. When Alex goes out on a errand with Nic, an old lady asks why she (a former hooker) is still hanging around with Benriya. Nic says because "She's my woman." He's lying for the sake of simplicity though.*
Shoot the Shaggy Dog: In chapter 20, a Twilight is forced to attack the heroes to save his family. He ultimately fails and dies in his mission but as it turns out, his family was killed (brutally) before he even made it to his destination. Harsh.
Doug underestimates Delico, who is a D-level Twilight because Doug himself is an A-level. This is played with because while Doug is mostly right, Delico manages to disrupt Doug's attempts on Monroe's life long enough for Nicolas to show up.
Where The Hell Is Ergastulum?: In the beginning it seems as though all the action is happening in a Mediterranean country, with Italian (especially) and Greek naming conventions prominent; however, there are odd names out, such as Cody, and in chapter 8 a man is seen passing what look like American hundred dollar bills, with the vague profile of Ben Franklin on them. Then it's finally revealed that Ergastulum is its own city-state on/by an unnamed continent, with its own history. Not long after that, it's heavily implied to be either part of an alternate history or possibly even part of a completely different world from Earth (this is not entirely clear from the get-go, considering the otherwise realistic backdrop).
It seems likely to be an alternate history (or perhaps future?), since there's mention of Alex possibly being part Chinese as well as black. Which would be an acknowledgement of China existing in the world.
Woman in Black: Alex, when she goes out with the boys on official assignments.
Would Hit a Girl: Nic suggested killing Alex in the first chapter. Worick talks him out of it. Implied to be what happened to the Twilight girl Veronica.
Would Hurt a Child: A rather disturbing amount of people in the manga show a willingness to hurt children, innocent or otherwise. Often for no reason at all other than the child in question being a Twilight usually.
You Have Failed Me: The B-level Twilight is killed by his boss for failing to kidnap Dr. Theo.