Sherlock about Mycroft (in "The Adventure of the Bruce-Partington Plans"): "You are right in thinking that he under the British government. You would also be right in a sense if you said that occasionally he IS the British government.
Mycroft draws four hundred and fifty pounds a year, remains a subordinate, has no ambitions of any kind, will receive neither honour nor title, but remains the most indispensable man in the country.
...His position is unique. He has made it for himself. There has never been anything like it before, nor will be again. He has the tidiest and most orderly brain, with the greatest capacity for storing facts, of any man living. The same great powers which I have turned to the detection of crime he has used for this particular business. The conclusions of every department are passed to him, and he is the central exchange, the clearinghouse, which makes out the balance. All other men are specialists, but his specialism is omniscience. We will suppose that a minister needs information as to a point which involves the Navy, India, Canada and the bimetallic question; he could get his separate advices from various departments upon each, but only Mycroft can focus them all, and say offhand how each factor would affect the other. They began by using him as a short-cut, a convenience; now he has made himself an essential. In that great brain of his everything is pigeon-holed and can be handed out in an instant. Again and again his word has decided the national policy. He lives in it. He thinks of nothing else save when, as an intellectual exercise, he unbends if I call upon him and ask him to advise me on one of my little problems".
Dr. Watson about Mycroft (in "The Adventure of the Bruce-Partington Plans"): "Heavily built and massive, there was a suggestion of uncouth physical inertia in the figure, but above this unwieldy frame there was perched a head so masterful in its brow, so alert in its steel-gray, deep-set eyes, so firm in its lips, and so subtle in its play of expression, that after the first glance one forgot the gross body and remembered only the dominant mind".