Number Two for Brains

Number Two (slang noun)
 * 1) The right-hand man, The Lancer/The Dragon, the second-in-command.
 * 2) Feces.

Number Two for Brains combines both meanings: the first applies to the position, the second to the skull's contents.

In other words, this is when the one who has authority over the others, gets orders from and reports directly to the boss, and has to keep everything running smoothly is a moron.

Usually Played for Laughs and a true sign that the guy in charge is truly Surrounded by Idiots. Compare Brains and Brawn, where the (possibly) stupid sidekick doesn't hold a position of authority. See also The Dragon, The Lancer, Number Two, The Igor, and Pointy-Haired Boss. May be The Starscream, but is virtually never Dragon-in-Chief—unless his boss is even more of a dingbat. Very often falls under Dumb Muscle. Contrast Hypercompetent Sidekick, which often diametrically inverts this trope by being paired up with an inept boss instead.

Film

 * Dark Helmet from Spaceballs is none too bright. He is also really short.
 * Dark Helmet's own Dragon, Colonel Sandurz, is arguably an inversion of this trope, as he seems to often be the only one with half a brain in the whole Spaceball fleet.
 * Kronk in The Emperors New Groove. One of the most lovably stupid lackeys of all time.
 * Otis in Superman.
 * Hedley Lamarr's flunky, Lyle, in Blazing Saddles.

Literature

 * Sergeant Colon in Discworld, in Guards! Guards!. Maybe not actively stupid, but certainly not bright and on top of things. And a disaster as Captain in The Fifth Elephant.

Live-Action TV

 * In Weeds, U-Turn spent a great deal of time chiding his friendly but incompetent underling Marvin, eventually

Radio

 * Moriarty in The Goon Show is the dumber member of the (usually) villainous duo, with Grytpype-Thynne as the brains.

Video Games

 * The sequel to Uncharted gives us, who pulls a Face Heel Turn early on, and demonstrates an outstanding lack of common sense more or less continuously, as well as deduction and decryption chops that would not shame a watermelon (although it's a bit unfair when the competition is Nate Drake). Sadly for him, the Big Bad is also a Bad Boss.

Western Animation

 * Greasepit from Biker Mice From Mars. Just why Limburger puts up with that oaf is anyone's guess.
 * Scorponok fits this trope to a T. He was really kind of an idiot; really the only things he had to his name were his loyalty and some surprising talents with inventing (though when his inventions went wrong, they went wrong.) To be fair, his loyalty certainly was a commodity what with the likes of Terrorsaur and Tarantulas on board.