One of the interesting things is that almost every character thinks that Contractors and Dolls have no feelings, when it is established early on that they DO have feelings. Huang's belief can be explained by his traumatic experience with a contractor earlier on in his life.
Throughout the series, the Gate is a puzzling aspect and most questions remain unanswered. Nobody even knows how the Gate appeared in the first place. The plot, however, is not the most important aspect. The characters are.
Hei is a ruthless assassin who earned himself the nickname of the Black Reaper because he was able to kill Contractors without any Contractor powers. In the first series he takes on various covers such as a university student, and he also works at a restaurant. It turns out that this job is selected for him to complete his missions from the Syndicate which is mysterious and never revealed. He is in the cell controlled by Huang alongside Yin who is a doll. He's true personality is ruthless and violent, but throughout the series he becomes more caring. Yin is uncommunicative at the start but gradually shows her love for Hei through various ways by the end of the series and more obviously in the OV As
Huang was dumped by a woman he loved (she was a contractor), possibly turning him into the bitter old man he was in Season 1, but towards the end he meets with her again and finds out the truth, and ends up sacrificing his life for the Contractors for whom he had expressed disdain earlier on in the series.
Season 1 develops the relationship between Hei and Yin, with them becoming closer to each other nearing the end and into the OV As
, but unfortunately interrupted by the emergence of Izanami. Hei becomes a drunk after he loses Yin, showing what an impact it had on him.
The fight scenes are well animated and without the dialogue that plagues Shonen anime, they are fast-paced and exciting because the outcome of any battle is uncertain.
Kirihara is a policewoman who is intent on finding out the truth, her interaction with Hei in Season 2 reveals his fiercely protective character. Suou is a girl who comes into the hands of Hei in Season 2 and matures under his training. Unfortunately she is taken away along with July at the end of Season 2 into Earth 2.0.
All in all the characters in the series are mature and interesting. Plot could be better though.