Mike Millar (played by Joe Spano) was a member of the National Transportation and Safety Board.
He led a group of Go Team members to investigate the crash of Flight 549. He did not agree with Mulder's theory that aliens were involved in the crash, and mocked him for it at the initial meeting. However, he soon found out that there was more to the crash than first seemed, and he opened up slightly to Mulder's ideas. He met Louis Frisch when Mulder and Dana Scully brought him to him, but could not accept the story they had. Then, when he found Sharon Graffia after she was returned from her abduction and saw an alien spaceship stop above him, he listened to Mulder's story on the crash. He seemed to be unsure of its truth (although he did not exactly call it a lie) but didn't want to report it to his superiors. (TXF: "Tempus Fugit", "Max")
Mike Millar was kind-hearted, and, according to Scully, was "the one man who truly wants to figure out what downed that plane". As such, he was very careful with his job, and at the end of the investigation, thanked each of the team members, telling them they had done a great job.