General Thomas Callahan was the commanding officer at Fort Evanston Military Hospital, Maryland. He served in the Gulf War. (TXF: "The Walk") At first, he was resistant to Mulder and Scully's investigation but when Captain Janet Draper is killed he becomes more open to them and tells them about strange happenings at his house. (Receiving whispering voicemails)
When the MP guarding his house steps away for to smoke his son Trevor is buried in his sandbox and suffocated by the astral projection of Leonard Trimble. Although not shown his wife is also later killed by him as well. Leonard attempts to bait the general into shooting and killing him but the general resists.
Although unconfirmed it is possible the general denied or had a hand in denying Leonard's family's request that his body be buried in Arlingon National Cemetery as there was no evidence of Leonard's involvement in the many deaths.