Sidney Kidd--
In the film, he is the publisher of "Spy Magazine", who sends Jimmy Stewart and Ruth Hussey to Katharine Hepburn's house to do a story on her, while Cary Grant tags alone. In the film he is played by Henry Daniell, who played Professor Moriarty to Basil Rathbone's Sherlock Holmes once. He was also the main villain in Errol Flynn's "The Sea Hawk".
But for anybody who has seen a stage production of "The Philadelphia Story"--does Sidney Kidd appear in the play? I have read a cast list for the stage version, in which the character is not included. And he never appears in the musical remake, "High Society".