Fair for Its Day: Some of the dialogue between men and women can read as incredibly chauvinistic, but this has to do with the time period the books were written in. YMMV very much on whether or not Heinlein was actually purposefully sexist.
The Ghost: Mycroft Holmes aka Mike the supercomputer. He is spoken of repeatedly and admittedly the whole reason the operation with Richard is put into action. He never says a word, or is seen "on screen." This even follows through to the sequel To Sail Beyond the Sunset, where he is reportedly given a human body and wedded to the Long family. Again, never seen.