The Consigliere

Elanor's work at Dover was considerable. There was a continuous flow of messages to and from the Countess who was busy supporting her husband's revolt against the King. She dispatched assistance to him including a certain William the Engineer, who was evidently a specialist in siege weaponry. She procured military equipment for her husband's use, maintained relations with a substantial body of military followers and conducted negotiations with various third parties. Among the visitors who dined with Elanor of Dover, were leading citizens from the port towns of Sandwich and Winchelsea, whose support or assistance she was presumably trying to enlist, and she fostered relations with visiting ambassadors and merchants from overseas...

—Daily Life In Medieval Europe by Jeffery L Forgeng

The Consigliere is a Number Two who is the Number One's voice of reason. Their advice is valuable to the boss because they're usually right, which puts them in a unique position to "Speak Truth to Power" when other staff might be scared to contradict the boss. This kind of job security is largely self-reinforcing - the boss won't fire him because he needs his uniquely honest advice, but he only has the freedom to be honest because he knows the boss won't fire him. Often the smartest guy in the room, or just a calming influence to a Hot-Blooded boss. There's a high likelihood of his being a retained lawyer.

Similar to The Obi-Wan, but usually the same age or younger than their Luke. Possibly, but not necessarily, the Only Sane Man. Might be The Puss in Boots if it turns out that he's actually the one in charge, or the Hypercompetent Sidekick if their boss would be useless without them. If he uses Obfuscating Stupidity to get away with Brutal Honesty, he's The Jester. Contrast with Evil Chancellor; compare and contrast The Good Chancellor.

Sometimes, he'd make a better ruler than the real one, but he doesn't want to be in charge for some reason. His boss may even recognise this, which could be a source of jealousy or mistrust.

This position isn't only reserved for men, mind you. It's not unusual for the wife of a powerful ruler or businessman to take on this role, particularly if she is a Proper Lady or Yamato Nadeshiko. This might overlap with the Lady Macbeth, if her husband is of the less-than-scrupulous character.

Named for the Mafia position made famous by Tom Hagen in The Godfather (so any self-respecting Don will keep one on staff as a matter of principle).

And in case you were wondering - con-sill-YEH-ree. Sort of. It's Italian for "counsellor."

Anime and Manga

In Baccano!, Maiza seems to play something like this role to the Martillo crime family.
In Neon Genesis Evangelion, there's one Kouzou Fuyutsuki...
A good part of the role of the kirin in The Twelve Kingdoms is to be an advisor to the Fisher King they choose.
- And a good part of the ruler's role is to ignore the kirin, since the kirin is inherently (and often illogically) compassionate, which is not enough to rule a kingdom.
Seigfried Kircheis is this for Reinhard von Lohengramm in Legend of Galactic Heroes.
Masa is the closest thing to this for Sun's dad Gozaburo in My Bride Is a Mermaid. Had the show been more serious, it would have been more obvious.

Comic Books

Star Wars Legacy: Darth Wyyrlok III was this to Big Bad, Evil Overlord and Galactic Emperor Darth Krayt, until he pulled a The Starscream.
- Wyyrlok is a particularly interesting case as he had no interest in power. He realized that his master's degenerative condition was driving him insane, and killed him so that he could become an immutable symbol rather then a flawed man.
- Moff Nyna Calixte, the Only Sane Man of the Moff Council, was also this to Morlish Veed. Until she betrayed him to spy for Roan Fel. Veed found out and apparently killed her.

Film

Tom Hagen in The Godfather, and any Mafia "consigliere" in general, since that is the job description.
Another Tom, in Miller's Crossing, who doubles up as The Dragon.
"Number Two" in the Austin Powers movies (with his "Virtucon makes millions legitimately" speech in the first movie)
Michel in Dodgeball, who White Goodman calls his "fitness consigliere".
Ship's doctor Stephen Maturin occasionally plays this to Captain Jack Aubrey in Master and Commander, being a close personal friend.

Literature

Davos in A Song of Ice and Fire, to King/Lord Stannis. Of course, Melisandre kind of nullifies all his advice, but he tried—and he used to succeed. Stannis chose him specifically because of this trope. He was so brutally honest that his lord appreciated how he'd always say the truth no matter what.
- Actually, Melisandre also plays this role for Stannis, and her advice is pretty much always right.
- Eddard Stark serves this role for his friend, King Robert.
- In general, the Hand of the King was supposed to serve this role for his King. Various Hands had varying degrees of success.
Jeffrey Pelt is this to the President in The Hunt for Red October.
John Sutter in The Gold Coast and The Lake House ends up with a Mafia don as his friendly next door neighbor, who co-opts him into this trope. His son tries the same thing in the sequel.
Before the Interregnum, some of the most important nobility Dragaera had Discreets, who are something like therapists crossed with advisers. Too bad Adron's Disaster killed all of them except Pel.
Mary Ann Patten in The Captains Wife (which is Based on a True Story)
Dirch Frode, family lawyer for the Vanger clan, to patriarch Henrik Vanger, in The Girl With The Dragon Tattoo.
Oxley is this to Father Thames in Rivers of London, in the court of Mama Thames Tybur...Lady Ty likes to cast herself in this role, but is really The Starscream.
Thurfir Hawat and Lady Jessica in Dune are this to Duke Leto.

Live-Action TV

Lord Melchett in Blackadder II.
- This only rarely applies, however. In reality, Elizabeth only keeps "Melchie" around because of his simpering, shameless toadying and readiness to agree with her at the first sign of hostility. If he stopped being such a suck-up she wouldn't hesitate to have him executed.
Stringer Bell in The Wire - something of a Reggie Kray parallel in the way he tries to calm down his Hot-Blooded gangster boss, and shows impressive business skills when that boss is put in prison. Also Norman to Carcetti, who is kept on staff because of his willingness to "speak Truth to Power".
Lost's Richard Alpert is apparently a permanent consigliere (a term used by the showrunners) to the ever-changing leader of the Others.
- Actually, they described him as someone who is not interested in leading the Others but is very influential in finding and selecting a leader. He is described as being similar to a Panchen Lama choosing the next Dalai Lama. The scene where he shows a young Locke a bunch of items and asks him which of them is his is a direct reference to this.
Silvio Dante in The Sopranos
Eric often fulfills this role to Vince in Entourage.
Leo McGarry in The West Wing to President Jed Bartlet, both as Chief of Staff in seasons 1-6, and as an advisor later in S6. Invoked when Leo calls himself a "wartime consigliere" to Toby in "17 People".
S.P.O.C.K. The Pothole says it all.
Wilson in Dad's Army ("Do you think that's wise, sir?")
Though he started out more as a jerk, David Rossi in Criminal Minds has become the consigliere to Hotch, and to a lesser extent, Morgan.
Nick George for Tripp Darling, and by extension for the whole Big Screwed-Up Family, in Dirty Sexy Money. His father had the job before him.
Mrs Onedin in The Onedin Line is this for her husband's firm. She is loyal, determined, and Good with Numbers . James Onedin married her for a ship she owned, and started to love her because she was so canny a businesswoman. Who says money is unromantic?
- She is obviously a Shout-Out to the many shipping wives of the time who had an unofficial but very real power over the business.
Merlin and Gwen tend to act as this for Arthur, especially after he finally becomes King of Camelot. Although Arthur's acceptance of this tends to Zig-Zagging Trope a fair bit.
Boardwalk Empire gives young gangster Jimmy Darmody two consiglieres; his mother, Gillian (who is also a Lady Macbeth), and Leander Whitlock, a Cool Old Guy who also advised his father.
- While we haven't seen too much of it yet, Meyer Lansky will become Lucky Luciano's. At this point, they're more Those Two Bad Guys and Co-Dragons to Arnold Rothstein.
The Borgias: Niccolo Machiavelli to the Medici family. Yes, that Machiavelli.

Tabletop Games

Shadowrun. The consigliere of the Mafia Finnigan family of Seattle is Albert "Uncle Al" Cavalieri.

Theater

Iago to Othello (though he abuses his position of trust).
- Maybe more of a Starscream, as he's using that trust to destroy Othello.
Hagen in Götterdämmerung pretends to be this, but he's actually The Chessmaster who suffers in Chronic Backstabbing Disorder.
- Tom Hagen from The Godfather is probably named thus as a Shout-Out.

Video Games

Mafia II has Leo Galante, consigliere to Frank Vinci, head of Empire Bay's older and more traditional family. He helps Vito out after they meet in prison, and their friendly relationship continues after both of them get out and Vito becomes a made man. It helps Leo after a hit is put out on him a bit later, which Vito rescues him from, and hit helps Vito get out of the shithole situation he gets himself into after Henry's drug deal goes wrong much later. Too bad that it doesn't help Joe.
Katakura Kojurou to Date Masamune in Sengoku Basara. It also helps that the otherwise Blood Knight-ish Masamune recognizes that Kojurou is the one man in the Sengoku era who could unfailingly kick his ass every single time if sufficiently motivated.

Web Comics

Though they share the same amount of power, Brian often acts this way for Angelo in Our Little Adventure.

Web Original

Augustus Wynn serves this role to Cassidy Cain in Grandmaster of Theft series, playing Blue Oni to her Red Oni & serving as a calming influence to her more hotblooded moments in addition to being her Battle Butler & Right-Hand Hottie.

Real Life

These Guys.
Arguably, Reggie Kray to Ronnie, who was known to (attempt to) persuade his brother out of his more Psychopathic Manchild tendencies.
Albert Speer was probably the closest to this trope in Nazi Germany. He was one of Adolf Hitler's most trusted and competent (and sanest) friends, and the man who strengthened Germany's war effort from 1942. Germany would have probably lost World War II sooner without him.
Vyacheslav Molotov would fit this trope during Josef Stalin's government.
Surprisingly not true for most real-world Mafia families. Consiglieres (the position actually does exist) are almost always low-level soldiers with legitimate or near-legitimate backgrounds who can represent the boss in non-criminal matters. For bosses in the post-wiretap era, their "advisers" are almost always shameless ass-kissers.
Di Renjie gained a reputation for this in the Chinese imperial court in the seventh century. The early part of his career could be summed up as a cycle of "Speak Truth To Power, get demoted/arrested, be proven right, get re-promoted," but later became so respected for embodying this trope that the empress usually referred to him as the State Elder.
Medieval Noblewomen would often serve as this as the above quote indicates.