Ferguson is the comic villain of Major Pettigrew's Last Stand. He's the Dr. Claw to the Major's Inspector Gadget, the Boris Badenov to his Bullwinkle. We use the term villain loosely here, though. He's not going to shoot the Major, but he is going to offend every sensibility the Major has. Shooting him might be less painful.
Let's get the villain part out of the way. Ferguson is the mastermind behind Lord Dagenham's plan to develop his land into a series of luxury estates, thereby enhancing the Downton Abbey-esque economic divide of Edgecombe St. Mary. He wants the Major's guns at any cost, even telling Roger he'll hire him if he can convince his father to part with him. That might be the highest price of all—working with Roger every day.
This plan of his falls through when he decides to marry Dagenham's niece, and Dagenham decides not to sell his land. Whew.
Despite potentially ruining the countryside, Ferguson is always good for comic relief, mostly in the way he acts and dresses. Like Sandy, the other American, he doesn't have refined British manners, for example calling Lord Dagenham Double D. Yeah, we're cringing, too.
He's also terrible at shooting, as demonstrated when he takes down one of the Major's ducks and has to apologize for it. Making matters even worse for him, and for everyone else's eyeballs, he's seen in an "unfortunate pink golf shirt," on the course (6.96), he looks like "a circus barker" (15.39) at the shoot, and the one time he dresses nicely, he enters the costume party "checking text [sic] on his phone" (17.11), which totally spoils the illusion.
This all may be funny, but it's important to remember that no one in the upper class has a problem with Ferguson. People have a greater problem with Mrs. Ali, even though she's not planning to change the entire town. But Ferguson can pretty much do whatever he wants because he is rich and white, even though he's an outsider. That's no laughing matter.