In Freeside, near the gate to North Vegas Square, you will find two bodyguards for hire and a Kings gang member standing on the street. Pay the bodyguard for hire 100 caps and he will follow you through Freeside until you leave. The other one is Orris, who charges double. You are required to expose his scam or kill him in the quest G.I. Blues.
Killing the bodyguard will impact negatively on your Freeside Reputation.
The bodyguards rotate, changing which bodyguards you will see upon entering Freeside, it will always however be one Kings gang member and Orris until you complete G.I. Blues, and it is unknown when the rotation happens, just that it results in different bodyguards being at the gate.