Older
brother of
Sherlock Holmes, and his equal or better in mental power. However, Mycroft is extremely lazy and won't go outside of his
home,
work or
club, even to research a
case, so he calls in Sherlock when he wants
legwork done, and Sherlock comes to him for government-related information because of Mycroft's job.
Also a self-aware computer in Robert A. Heinlein's The Moon Is A Harsh Mistress, who named himself after Holmes' brother, but is usually called Mike.