Chris is a sentry at the Navarro gas station, posing as a gas station attendant. His role is to dissuade travelers from proceeding towards the base, as well as receiving recruits from other Enclave bases.
Upon reaching Navarro, Chris will engage the Chosen One in conversation and attempt to dissuade him from investigating further. 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 inside the base to be hostile towards the Chosen One and shoot him on sight.
*However, if the Chosen One asks Chris about the Enclave, then express interest about joining and passes a Charisma check, he can enter the base unharmed.
Another good way to get past him is to just sneak behind him and kill him. This way he also won't inform the Enclave.