Number Two (slang noun):
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 sure 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, Pointy-Haired Boss, and Underling with an F in PR. 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. If the bad guy has two subordinates who happen to be idiots, then it's Bumbling Henchmen Duo.
- Kronk in The Emperor's New Groove. One of the most lovably stupid lackeys of all time.
- Game of Thrones: Ramsay. As his more intelligent and self-controlled father's Number Two, he makes several stupid decisions due to his sadistic and impulsive nature. He ruins Theon to a point that he can't be used as a bartering piece with the Greyjoys, flays the Ironborn reavers at Moat Cailin rather than honor the conditions of the surrender, flays lords who refuse to pay their taxes, and holds a large feast for himself when Winter is Coming. In general, his sadism and impulsivity prevents him from waiting or devising non-violent solutions and consequentially, Roose spends Season 4 and 5 having to deal with his bastard/son's messes.
- In Weeds, U-Turn spent a great deal of time chiding his friendly but incompetent underling Marvin, eventually pushing him over the edge.
- Moriarty in The Goon Show is the dumber member of the (usually) villainous duo, with Grytpype-Thynne as the brains.
- Uncharted 2: Among Thieves gives us Harry Flynn, who pulls a FaceHeel 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.
- Tropico gives us Penultimo, your second-in-command who is utterly loyal but not too bright.
- In Biker Mice from Mars, given the limited budget available to him from the council of Plutark, Greasepit was probably hired by Limburger primarily because he was cheap. It's sure as heck not because of any competence on Greasepit's part. A flashback shows that Greasepit got the job because he had a good agent, or at least that was Karbunkle's excuse for not getting anybody better.
- Beast Wars: 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.
- Since his selection is limited, The Brain from Pinky and the Brain has to rely on Pinky a lot in his schemes. This is frequently their undoing — almost as frequently as Brain's own overconfidence.
- South Park features Mimsy, sidekick to Nathan. Occasionally subverts it by being a little more sensible every now and then.
- Larry, Rippen's loyal but moronic henchman in Penn Zero: Part-Time Hero. There have been many occasions where Rippen would have easily won if not for Larry screwing everything up at the last second.
- Several episodes of Kaeloo have Stumpy act as a sidekick to Mr. Cat, and whenever he does this trope comes into use.