Black Cat Vol 4

Black Cat Vol 4 by Kentaro Yabuki. A Shonen manga rated Older Teen, published in English by Viz Media in 2002.

Train Heartnet was once Black Cat, the most feared assassin of the secret organization Chronos. Now he travels as a carefree bounty hunter with his partner Sven, but his past catches up with him in the form of an old enemy.