Harold Callahan

 was a magnum toting Pentagon internal security officer and member of the US Army in 2077.

Background
While serving as a member of the Pentagon's internal security, Callahan recorded several incidents in his personal Terminal, locked Hard.

Appearances
is mentioned in terminal entries in the Fallout 3 add-on Broken Steel but does not actually appear in any game.

Behind the scenes
The character (along with his Magnum sidearm) is a reference to Harry Callahan, protagonist of the Dirty Harry series of movies.