Happy, played by Iron Man and Iron Man 2 director Jon Favreau, was used mostly for comic relief in the Iron Man trilogy. He's one of Tony's closest friends, and also works for him as chauffeur, personal assistant, bodyguard, and then chief of security at Stark Industries. Though he didn't have a big part in the films, he was a very enjoyable character and it's easy to see how Marvel could structure a One Shot around him. The short film could act similarly to the One Shots featuring Agent Coulson, as a "day in the life" scenario that puts the character in a funny, action-packed, and unexpected situation. We saw Happy's detective side in Iron Man 3, as he follows one of the Extremis-infected henchmen. Perhaps he could do something similar in the short film, set sometime during The Avengers, or between two of the other films? It would be fun to see what Happy was up to while New York was under attack. If the filmmakers turn to the comics for guidance, Happy has some interesting stories there as well. He was married to Pepper Potts at one point (though I can't see that happening at all on screen), and even occasionally turns into a monster known as Freak.