This character is the legendary number XIII of the Chronos numbers in the anime Black Cat.
What we know about Train's past is that his parents were killed by a number, an assassin from Chronos. That number took Train in and cruelly taught him how to be an assassin too. It would appear that Train eventually paid the man back in full, though, because Train's master later died by a bullet, fired from Train's own gun. From then on, Train was an assassin that never failed, until after he met Saya. This girl had managed to undo his assassin nature entirely. And when she died, he fled from Chronos.
At first, we see a side of Train that's quite possibly as bad ass as they come. His speed is unmatched, his demeanor, cold and heartless, and his marksmanship, unrivaled. The only people willing to get within arms length of him are the Chronos council, Chronos #1 (his female commander), Creed (a Chronos number and male psycho lunatic, who's creepily infatuated with Train), and Saya (a female sweeper that he met one night, and probably the main reason behind him running away from Chronos).
To make a long story short, Train and Saya became friends secretly. Creed found out. So when Saya asked Train to watch fireworks with her, Creed showed up and killed her instead. When Train showed up, it was too late to save her. It was around that time, or slightly before, that he met Sven and Eve.
Train ditched Chronos and became a sweeper with Sven. The interesting thing is that Train is probably the complete opposite of his murderous self when he's not in a fight. When Train becomes a sweeper, he's lazy, gluttonous (eating everything in sight), and in the general word, totally apathetic. But then he gets a sweeper job, and he gets caught up in the mess. Occasionally, when battles go poorly, he'll still revert to his old assassin demeanor of a self. Still, its a solid statement that he hasn't killed anyone since he left Chronos. This of course stands in entirety, but when he faces off with other numbers, his speed and skills don't appear to have actually decreased at all in his time away. And his selfish attitude hasn't changed either.
He and Sven often get into word battles, and Train loses most of the time. This usually holds true until Sven acts stupidly... then Train promptly hits him.
As a character Train appears selfish and pretty apathetic. This is probably completely reasonable when you look at the past he's had. Even through struggling to accept Sven and Eve into his life, he isn't above listening to a few words of wisdom, no matter the source. While he's taken on a path of repenting for his ways as an assassin, it can also be said that he still finds a use for the skills he learned as a number. While many of his emotional scars from his past have healed with time and training, the one related to Saya never really does. He is a mysterious shadow that passes through the night, but he also understands the deeper, darker dealings of the world, even if he wasn't really meant to...