So, what's the concept behind him? Or story rather?

He was a metalworker, making honest pay at a shop, when he was entered in this contest thingie. He ended up winning the prize, which was a small cash thing and the chance to participate in one gladiator fight that the company also owned. If he won the fight he'd get a four-year multi-million dollar contract with them, and win the full final prize as well.

He decided to go for the fight, figuring it was at least worth the shot, and got put against this absolutely huge bear wielding a big ol' hammer. He was given a pair of Kama on chain, the company figured he would have no bloody clue what to do. Luckily for \ he had done quite a lot of poi-type stuff with glowsticks, etc, so he figured this wouldn't be too different. He was right, ended up winning with a few good hits to the bear's back. This earned him his name, Backslash, and all the prizes too.

So yeah, he participates in a variety of gladiator fights, ranging from first bloods to incapacitation, with lethal and less lethal thingies... yeah. Company wasn't too pleased at first but the crowd likes him. Originally he was from a world where death could be made non-permanent under certain conditions or in certain areas.

As to the last part, "originally" as in this is no longer cannon?

Well, it just depends on the context I have him in. He's evolved quite a bit since then, much less of an asshole now. It's just a bit more difficult to have him have this illustrious fighting career without some pretty nasty wounds with any mroe realistic setting