Train Heartnet
Voiced by: Takashi Kondō (Japanese), Jason Liebrecht (English)
Also known as Black Cat and the main protagonist in the story, he starts as a ruthless assassin but learns compassion from Saya Minatsuki. After his change of heart (after he met Saya) , he works as a sweeper alongside Sven and Eve. He always wears a bell around his neck to remind him of what he once was and could once more become. He has a gun made just for him by Chronos, called Hades. He also has a soft spot for women and children, especially when tears are shed.

Sven Vollfied
Voiced by: Keiji Fujiwara (Japanese), Brandon Potter (English)
A sweeper and Train's partner, he befriends Eve and treats her like a person instead of a weapon. He, Eve, and Train work together. He has an eye that can see into the future(which he got from a friend who sacrificed himself to save Sven), but it either puts a strain on him or drains all his energy.

Eve
Voiced by: Misato Fukuen (Japanese), Brina Palencia (English)
A genetically engineered bio-weapon, she uses nanotechnology to transform her body. Raised like a lab rat, Sven treats her like a real person and opens up her world. She tends to follow Sven, and since she and Train are so close, treats him as her rival as a Sweeper (although more so for Sven's attention). Eve later looks up to Sven as her father. She is often called "princess" by Train. She also has photographic memory. In nearly every episode, she is seen reading a new book. It becomes a sort of gag in early episodes as she read that putting bells on bad cats will make them good, thus forcing one around the neck of Train.

Rinslet Walker
Voiced by: Yukana (Japanese), Jamie Marchi (English)
A world class thief also known as Rins who she works for everyone, from the government to mob bosses. She claims she can steal anything and manipulate any man. She sometimes works alongside Train and Sven. She always comes out with money in her pockets, regardless of how it might have gotten there.

Creed Diskenth
Voiced by: Shinichiro Miki (Japanese), Chris Patton (English)
An Ex-Eraser of Chronos, and an admirer of Train. Creed is responsible for the death of Saya Minatsuki since he believes that she bewitches him. Years later, he forms a group known as the Apostles of the Stars to start a revolution on this world he considers corrupt. He meets Train again and request him to join their ranks to put an end to Chronos. His primary weapon is a sword called Imagine Blade which he can transform it to many appearance.

Saya Minatsuki
Voiced by: Megumi Toyoguchi (Japanese), Caitlin Glass (English)
Before her death Saya was an accomplished sweeper, whose fun loving spirit and easy likability made her Train Heartnet's first real friend. Her involvement with him lead to his more human outlook on life and his departure of the Chrono numbers.
Due to how close Train and Saya were, Creed grew jealous of the "Witch", saying that she was clouding Train's mind. Creed murdered Saya with the use of his sword, the Kotetsu. She died in Train's arms during a fireworks show, the thing she was most looking forward to see. While she was alive she wore a kimono, just because she thought she looked cute in it.

Sweepers
In the world of Black Cat, Sweeper is a profession that specializes in capturing criminals for rewards. Sweepers are not unlike bounty hunters, they are licensed by the government and free to move about as they please, however, unlike real-life bounty hunters sweepers are internationally accepted and endorsed by the governments of the world and respected for their contribution to maintaining justice. The pamphlets with information on criminals are spread throughout the world in public places like bars, where Sweepers hang out.
Sweepers vary in their appearance and style of fighting, and many times conflicts erupt among themselves when two or more sweepers go after the same target. The criminals that sweepers have to capture are classified in levels, which range from F to SS, F being the least dangerous and SS the most dangerous. Legendary criminals who may never be captured are considered SS, the most violent and dangerous kind of criminal. An example of the SS is Creed Diskenth, leader and the most powerful member of the Apostles of the Star.

Chronos
Chronos is the secret organization that rules one third of the world through underground business in various areas. Though despite being a shadow society most characters who spent at least some of their time digging into the underworld know that they exist. Incidentally almost all speaking characters in the series seem to know of Chronos and the Numbers. Most of the Chronos members occupy high positions, such as mayors, politicians, among others. To protect Chronos, there are assassins in the command of the three wise men in charge of Chronos, the highest of them being Willzark. Willzark commands a secret force of the best assassins in the world, sworn to protect Chronos, called the ChronosNumbers, which range from No. I to No. XIII (More information on the Characters Section).
Lower in the hierarchy are the Chronos Erasers, which are assassins in service of Chronos. An example is Creed Diskenth. Other members of Chronos play important roles in the story in the manga, such as Carl Walken, which is the mayor of a town that raised Train and asks him personally to take out a Taoshi who has been causing considerable damage in his city, Gyanza Rujike.
In the anime, the Zero Numbers send a swarm of nano-machine beasts to destroy the Chronos Headquarters while the Numbers were away on Craken Island, killing the three wisemen. Financial support from practically all sources is cut, but Sephiria, No. I, and the remaining numbers decide to continue in Chronos, although they will have to raise it from its ashes.

The Apostles of the Stars
Sometimes simply referred to by the characters in Black Cat as "The Apostles", they are revolutionary group led by Creed Diskenth. Their aim is to "clean the world of filth", that is to say, kill people who cannot achieve the Power of Tao. Naturally, every member of the Apostles of The Stars has a power of Taoshi origin, which are awakened through the Shinkitou, a drink only concocted by Master Taoshis, such as Shiki. Another member of the Apostles, Maro, is also from the forbidden Continent, and together with Shiki, they bear the most traditional Tao power, such as controlling and summoning bugs (Shiki), using ink marks (Shiki) and controlling gravity (Maro).
Other members of the Apostles rely on more "deviant" powers, such as Creed's "Imagine Blade" in the manga, which is a sword that can expand for miles depending on Creed's mental power, even though in the anime Creed's power is just the materialization of a small invisible sword, in replacement for his old Kotetsu, broken by Train. Other Tao powers include Kyoko's "Fire Breath", Charden's "Blood", Durham's "Magic Shot" or Echidna's "Gate".
The Apostles do not just wish to bring about a world revolution, but they seek revenge against Chronos. Creed due to his expulsion after killing Saya, and Shiki and Maro due to the war that happened between the Taoshi and Chronos long ago, which decimated the Taoshi from the Secret Continent. Although Tao has been the primary source of the Apostles' power, Creed also relies on Doctor's investigations on Nanomachines to grant him eternal power and elevate him to a god.

The Taoshi War
Many decades in the past, there was a war between the Taoshi and Chronos. Chronos eventually won, even though not all Taoshi were killed. The survivors among the Taoshi that make an appearance in Black Cat are no more no less than Shiki and Maro, which upon meeting Creed grant him the power of Tao in exchange for an alliance which objective was to destroy Chronos and bring about a revolution to this planet, which would choose the Taoshi among the people, and rid everyone else.
In the anime, the only Number in the current Chronos that witnessed this War was Mason, who himself became a Taoshi and was granted with the Sacred Technique. However, he had kept this from everybody in Chronos, planning the Eden project for a long time (More information below).
Nanomachines, Eve, Dr. Tearju, and Dr. Kanzaki
The nanomachine investigation by Dr. Tearju Lunatique allowed her to achieve the knowledge that might allow her to create a prototype of a human that could transform and eventually become the ultimate weapon. In the manga, Creed Diskenth financed the research of Dr. Tearju, hired by Torneo Ludman, to create Eve, which would be the perfect weapon for him to use, while in exchange Torneo promised to have Dr. Tearju research nanomachines in the areas that were of Creed's interest, such as the legendary immortality nanomachines.
Eventually, Dr. Tearju realized she couldn't do that and quit, for the world's safety, but due to blackmail from her former assistant, one of the Apostles, Doctor, or Dr. Kanzaki, she passed him the nanomachine research documents, which allowed Doctor to perfect them in order to make Creed a god. Worth noting is that in the Anime, Dr. Tearju didn't pass the documents on to Doctor, which stole them instead. In the anime, he also used them to produce the Eden machine that would work when combined with Eve.

The Zero Numbers and Eden (Anime Only)
The Zero Numbers are the Chrono Numbers gathered by Mason to aid him in the Eden project, which he planned since Eve was created by Dr. Tearju. The Eden project consisted of bringing "World Peace", which would actually consist of turning every single person on the Earth into creatures without mind of their own, only capable of obeying the Zero Numbers. The Zero numbers were also implanted with nanomachines that allowed them to be immortal within Eden's energy field, created by Doctor.
For Eden to work, however, it was needed that Eve entered it, in order to combine with the program inside Eden, Adam, to put into motion the giant creature, which was Eden combined with Eve. Mason had gathered people from Chronos, from the Apostles of The Stars, in order to aid him when the time to retrieve Eve came. That is because Eve needed to be raised well in order to be "perfected", and since the day she met Sven and considered him the ideal "father", Mason decided it would be of best interest to the Zero Numbers that Sven and Train raised Eve, and they would retrieve her, like happened in the story.

Black Cat was published in Japan by Shueisha in Weekly Shōnen Jump from 2000 to 2004 and collected in 20 tankobon volumes. It is licensed in English in North America by VIZ Media, with 15 volumes published as of July 2008.[3]

Black Cat's official site (Japanese): http://www.j-blackcat.com/
TBS's Black Cat anime official site: http://www.tbs.co.jp/anime/blackcat/
FUNimation's Official Black Cat site: http://www.blackcat-tv.com/