Black Cat is a manga by Yabuki Kentaro, and spans 20 volumes. The manga is licensed by Viz, but hasn't been released in English yet. Black Cat is also now an anime, but it should be mentioned that the anime differs greatly from the manga (still has the main points and is entertaining though).

Black Cat is about an ex-assassin dubbed Black Cat with the number XIII tattoed on his chest. Black Cat a.k.a. Train Heartnet left a secret society named Chronos after an event that happened while he was in. Now he's travels around with his partner Sven Vollfied as sweepers (the term in the series for a bounty hunter). While conducting their regular sweeping business they take on a girl with nanomachine transforming abilities named Eve, and Train's old partner Creed tries to take over the world.

^ Tis a brief and confusing summary, read the series to understand. Sorry!

Black Cat has a lot of similarities with other series, such as Cowboy Bebop. Ex-syndicate guy that becomes a bounty hunter, old partner shows up and causes lots of headaches and deaths. Sound familiar? Well, Black Cat is it's own series and is quite good. I wouldn't read it if it was a Bebop clone, because it's not. Like Trigun with the reappearing black cat, there's a reappearing white cat. Lot's of similarities, but that doesn't make it a bad series. Read it and figure out for yourself. ^_^

Note that I'm using spellings for the characters names from the Matatabi character book. It's an official book and had the english spellings included, so I'm using them. I know quite a few vary from translations floating around the net, but this is what I'm using.