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.
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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.