Chris is a sentry and overwatch radioman at the Navarro gas station, posing as a gas station attendant. His role is to dissuade travelers from proceeding ahead towards the base, as well as receiving fresh recruits from other Enclave bases in the area of operation.
Upon reaching Navarro, Chris will engage the Chosen One in conversation and attempt to dissuade him from investigating further ahead. Should the Chosen One attempt to engage him in conversation or explore the shack behind him, he will state that there is nothing of value in the area and that he is trespassing on private property.
Attempting to push the issue by asking him about the Navarro base will result in Chris threatening to radio the Enclave and engage you in combat. Inquiring about vertibirds and informing him that the Chosen One is working with the Brotherhood of Steel will cause him to reveal how to access the base without initiating combat; however, he will still send a message to the Enclave via his radio which will cause the soldiers stationed inside of the base to be hostile immediately towards the Chosen One and engage him in combat sight.
If the Chosen One asks Chris about the Enclave however, then express interest about joining, and passes the required Charisma check, he can enter the base unharmed.
Another good way to get past him is to just sneak behind him and/or kill him. This way he also won't inform the Enclave.
As an alternative to the above, a player with a respectably high Steal skill might steal Chris's radio from him so that he may not alert the guards ahead.