K-51

Origin

Ken Flynn was the childhood friend of Rick Anthony.  The two grew up and Flynn became a secret service agent originally known as K-5.  Anthony became a mercenary.  He was so well known as such that even wrote a book about his occupation called Solider of Fortune.  The two would end up on opisite side of a war and Anthony would be executed with a bullet to the back of his head for refusing to fire on his friend.  K-5 would fire back at Anthony's executioner and leader of the rebel forces but it was little conciliation for a dead friend.
 
Later K-5 was switched to K-51. On K-51's first mission with the new title he would meet Z-19 whom he thought was an enemy spy but turned out to also be a secret agent and ally.  The two would take on a real spy and traitor Jack Coe.  K-51 was forced to shoot Jack Coe in a gunfight on-board a ship.  It was this mission that Z-19 met and stated having feelings for K-51.

Creation

K-51 was originally called K-5.  It was never explained why the name was changed.