Darker Than Black

Well...I don't have much information about this anime but...I like this anime nevertheless. Here is the summary I could write about it.

Darker Than Black is a story of a man named Hei and known as BK-201, an assassin. In the normal life, Hei is known as Li Shengshun, a gentle-hearted student from China. This anime/manga told about a life where contractors also lived. A contractor is a person who has certain abilities but they have to do something for the repayment whenever they used their abilities. For Example: Hei eats a lot of foods after he used his ability. But there is various way of payment.

Here is the summary from wiki:
Ten years ago, an inscrutable and abnormal territory known as Hell's Gate appeared in Tokyo, altering the sky and wreaking havoc on the landscape. The heavenly bodies disappeared, replaced by false stars. During this time, people who possess various special abilities emerged. Kept secret from the masses, these individuals, known as Contractors, are able to murder in cold blood. Various nations and organizations around the world use Contractors as spies and assassins, often resulting in violent battles for information. Each fake star has a corresponding Contractor, and they vibrate when their Contractor's power is being used. As such, Contractors are usually identified by their star's Messier catalogue number. However, their abilities are gained at the cost of their humanity. They are implied to lack human emotions which include no aversions to murder, and they are regarded as extremely rational and logical in their decision making. Contractors are thus named because of their need to always complete remuneration or payment each time their power is used (ie. eating flowers, breaking fingers, drinking blood). It is not entirely clear what happens to one if he or she fails to complete the remuneration.

Main character for this story are: Hei, Yin, Huang, Mao, Misaki Kirihara, and Shino Kana (in manga)

Anime Episodes: Episode 1-25 + 1 OVA titled "Beneath the Fully Bloomed Cherry Blossoms" (Completed)
Manga Chapters: Chapter 1-9 + Omake 4com