Pixel (or The Cat Who Walks Through Windshields) is the former cat of Robert Heinlein who joins the Callahan's gang during their move to Key West. Pixel has been seen to communicate with Erin and Ralph. Pixel can apparently transit which allows him to appear and disappear wherever he wishes.

A cat which may or may not have been the same Pixel previously appeared in works by Robert A. Heinlein (The Cat Who Walks Through Walls and To Sail Beyond the Sunset), and may also have later appeared in a short story by M. David Blake ("Absinthe Fish").