Anime: Darker Than Black

Huang, Yin, Hei, Mao. Yes, the cat is a main character.

"A night sky full of cries
Hearts filled with lies
The contract — is it worth the price ?
A soul pledged to the darkness
Now I've lost it, I know I can kill
The truth exists beyond the Gate!"
"Howling", Abingdon Boys School (the first OP)

A Fantastic Noir Science Fiction Anime and Manga series produced by Studio BONES.

Ten years ago, an inscrutable and abnormal territory known as Hell's Gate appeared in Tokyo (as well as a corresponding Heaven's Gate in South America), altering the sky and devastating the landscape. The heavenly bodies disappeared, replaced by false stars. At the same time, people who possess various special abilities emerged. Kept secret from the masses, these individuals are known as Contractors. However, their abilities are gained at the expense of their humanity: they lack most human emotions, particularly a conscience, and thus have no overriding loyalties or aversion to killing. Various nations around the world use Contractors as spies and agents, often resulting in vicious, no-holds-barred battles for information. They're called Contractors because each has a remuneration or obeisance (called "the price" in the Funimation dub) that their "contract" compels them to perform after they use their powers and because they have no loyalties outside the scope of their current employment.

The story follows Hei, a Contractor working for the Syndicate in Tokyo, as he performs various tasks for them while trying to discover what happened to his sister Pai, who disappeared five years before the start of the series during a war fought over Heaven's Gate.

The first season, subtitled The Black Contractor, is available in some countries, on Hulu, YouTube and FUNimation's website. A series of OVA's with the title "The Black Contractor (Kuro no Keiyakusha) Gaiden, bridging the events of the series with the sequel, followed soon afterwards.

A second season, subtitled Ryuusei no Gemini, translated as Gemini of the Meteor, ran from October to December 2009. Funimation licensed the second season and released it in 2011.

There's a manga called Darker Than Black: Shikkoku no Hana (Jet Black Flower) covers events in Tokyo after the first season, but before the OVAs.

A shojo manga known as Darker Than Black is set with its own plot that does not have to do anything with the first season.

Trope examples go in the appropriate sub-pages.