His name should be "Ken", not "Ken Masters". You don't call Ryu "Ryu
Hoshi" or Mario "Mario Mario" and just because Ken has a memorable and
admittedly cool last name, you don't call him "Ken Masters".
And who put Kobra as a friend of Ken? Clicking the link reveals a
potential long-lost twin, but my first thought was GI Joe Cobra.
I disagree stongly. You have a first and a last name, but people probably usually just call you by the first name. Doesn't mean you wouldn't want your last name on your profile page.
The reason to why Ken's name is Masters is because Mettle, the company who created Barbie, sued Capcom for having a character with the name Ken. So, Capcom decided to name Ken, Ken Masters. And about Ryu's last name being Hoshi is all wrong. The name Ryu Hoshi was used in the movie Street Fighter: The Movie. The last name itself is non-canon, as in, it isn't real in anyway. Hope this cleared things up a bit.