Apart from the obvious - they should be cheerful individuals and not be prone to either excessive whining or psychokiller rampages - good travel companions must show a proper appreciation for the important things in life. These include but are not limited to the following:
Musicals involving witches. Because obviously.
Nerd bookstores, particularly those with a plentiful supply of Doctor Who toys and hard-to-find fantasy books.
Dead people. Good travel companions will eagerly spend hours strolling amidst the bones of such as Sir Isaac Newton, Charles Darwin, George F. Handel, Henry Purcell, and Charles Dickens. They will also agree to a thorough inspection of the Egyptian mummies, even if that is the most crowded room in the entire British Museum.
It's perhaps a little non-obvious, but the best travel companions have relatively little sense of direction. Excellent sightseeing can be done when you strike out in one direction or another (neither of you knows which), with no particular destination in mind and no knowledge of how to return to your starting point.
And of course - particularly when visiting London - a good travel companion should have a healthy respect for blue police boxes, combined with an understanding of when and where it is appropriate to use a sonic screwdriver. That is, all the time and everywhere.