Who is he
Some time ago, bitcoin owners decided that allowing a centralized exchange with a completely trusted CEO to hold their money for them like a bank would be a good idea.
Yeah, I know, it's exactly the sort of thing Bitcoin was intended to prevent, but ... reasons!
Well, predictably the banker's business model involves fractional-reserve banking.
And as intended, Bitcoin is not compatible with fractional-reserve banking.
So Mt Gox (you pronounce that, EMPTY-Gox) imploded and the sort of people who trust central bankers lost money. Even though they had deluded themselves into thinking that they weren't trusting central bankers.
(cue world's smallest violin, playing a sad song).
Mark Karpeles? Yeah, he was the central banker in this scenario. Last I heard France and Japan were negotiating whether France could extradite the guy and prosecute him there on a different set of charges. But that was, like, a year ago.