His name is Barnaby Jones.  He's a British Junior diplomat that works at the British Embassy in Pyongyang. 

Source: http://www.dailymail.co.uk/news/article-2182429/Junior-British-diplomat-identified-mysterious-Westerner-pictured-riding-rollercoaster-North-Korean-leader-Kim-Jong-Un.html

Edit:  Oh good lord, I get it.  Barnaby Jones is everyone's new best friend.  