- "Right, you're the control, and if that fails, I'm the damage."
- ― Jack Campbell to Elliot Swann, shortly after arriving on Mars [src]
Jack is seen with Elliot Swann in some of the cutscenes, although during gameplay, the player gets a chance to watch him blast a group of Trites through a window using the BFG 9000 at very end of the Alpha Labs Sector 4, just before the first contact of the Vagary.
Jack Campbell is Counselor Swann's loyal bodyguard, a former Marine of few words who lets his BFG-9000 do the talking for him. A master of every significant form of armed and unarmed combat, Campbell is a born survivor for whom adherence to duty is everything.
If Campbell has an opinion of Swann's mission, he doesn't share it. It's not his place to do so. As Campbell himself points out, they are there to conduct damage control: Swann's the control, and Campbell's the damage.
Jack considers himself to be the "damage" part of "damage control" (Swann being the "control"). Wielding his BFG 9000, Jack Campbell is the bodyguard of Counselor Swann and protects him while they attempt to survive the demon invasion. Jack does not have a high opinion of Doctor Malcolm Betruger, who he considers to be stubborn and correctly does not think that he will bend in to any of the UAC's demands and that Swann's mission has no point.
In the pre-invasion Mars City, he is there in a separate conference room with Betruger and Swann. While they are arguing, he notes the player's presence by looking at him. After the cutscene involving Swann and Betruger, he can be seen walking out of a door after Swann.
Shortly after the invasion, when Swann's suspiciousness is confirmed regarding Betruger's involvement, Campbell then resorts to "plan B" which is using brutal force.
While seen mostly just in cutscenes, he is seen ingame several times: In the Administration Complex's Global Network Terminal with Swann; running down a hallway with Swann in Alpha Labs Sector 1 while the player battles some Maggots in the Hydrocon Chamber; again when he kills some Trites at the end of Alpha Labs Sector 4; in the Communications Room, he uses the BFG to destroy all the computer equipment; and for the final time when he encounters the Marine. He is badly wounded, and advises the Marine to watch out for Sergeant Kelly (Now Sabaoth), who took his BFG 9000. He dies shortly afterwards upon muttering the words: "Sarge... find him... you gotta find him... my gun... he's got my gun...".
- He has the same body armor like the Marines commanded by Sergeant Kelly. However, he never takes orders from him.
- Campbell can be killed in Alpha Labs 1 when you shoot the Hydrocon.
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