Dr. Davey.
Christopher Davey was a scientist working at Polarity Magnetics alongside Dr. Chester Ray Banton. Although he pretended to be Banton's friend, he betrayed him to X, intending to turn him over to X for study.
However he got more than be bargained for when X (or one of his operatives) shot him and killed him, and then switched his body out for Banton in the room with Polarity Magnetics' particle accelerator so that when the machine fired it would destroyed Davey's body, making Mulder and Scully think it'd been Banton who'd died in that room and not Davey.
But sacrificing Davey wasn't without its setbacks, as a government scientist working with X in studying Banton complained that Davey would've been useful to him in his work, and his death would set their research back for some time.