Geoffrey Callaghan is the head of British Military Intelligence.
Being in the Army himself, Callaghan holds the rank of Colonel as he leads what amounts to the total of intelligence gathering for the British Empire before the turn of the 20th century. His career there must have been quite interesting because he has from the time we first meet him the nickname of "Steel". Why is as yet unknown to us.
And that concludes everything I have learned about this man. Of the three adventures that were recorded, the second and third are as yet unavailable and the first one has a single copy for sale at a very high price.