All Contractors in Darker Than Black are said to be like this; part of their condition is that they always act rationally and with their own best interests in mind, dismissing emotional attachments. Throughout the series several counter-examples are shown, to the degree that whether the statement is actually true is up for discussion.
He acts like The Spock, but it should be noted that, mainly due to his own Pride, he disregards mounting evidence of Johan's existence until it becomes overwhelming. A true Spock would have impartially re-evaluated evidence and circumstances as they changed and based his judgements purely on logic. Also, his big fight with Roberto shows that he does have an emotional Berserk Button, and while his handcuffing himself to Tenma after being badly wounded is admirablyBadass, it could hardly be called a rational decision.
Exedore/Exsedol from the Macross and Robotech universes, though he has his emotional moments, too.
L from Death Note is a surprisingly good one, too. There's a minor deviation in that he sometimes informs people of his emotions, even though he doesn't display them unless the situation's truly dire. Near also qualifies. Mello? Not at all...
Ulquiorra from Bleach appears to be a villainous example of this trope- while his comrades generally act like children, he openly describes himself as an emotionless tool for Aizen to use as he pleases. The closest he ever comes to showing emotion are several cases where his eyes slightly widen and a single instance where he raises his voice.
Wilhelmina in Shakugan no Shana. There was even an episode called "Heartless Wilhelmina".
Sai from Naruto. He was raised in ROOT, which means he has no emotions whatsoever. Although he gets better at understanding them later when he joins team 7.
Kyoya from Ouran High School Host Club. He deliberately crafts a hardened, purely logical exterior, and gets pretty annoyed/confused when people (Tamaki, Haruhi) see through it. He's the one really running the club and making all the real decisions - slightly subverted, though, in that nobody fights him on his decisions since he never has to make life or death choices.
C. C. from Code Geass is a particularly snarky variant, at least while the cold side of her Sugar and Ice Personality is the one being shown. Far from being emotionless, she has very strong feelings, she is just able to keep them under control most of the time.
Piccolo of Dragon Ball Z…sometimes. He first appears as a raving, cackling lunatic hell-bent on killing Goku (justified, as he has all his father's memories, and is only a few years old.) Then he mellows, is shown meditating in a lotus position, becomes an Anti-Hero, and voila. Also, he's arguably the most intelligent character on the show. Sometimes, though, he gets angry. Like if Gohangets hurt. Then he goes from being The Spock to a Papa Wolf.
Fuu Hououji of Magic Knight Rayearth is an unusual example. She is much more calm, observant, and thoughtful than Hikaru and Umi , but she also applies her logic to a strong sense of compassion and attempts to comfort her friends.
Dr. Manhattan of Watchmen, who is referred to as "goddamn Mr Spock there" by a minor character at a cocktail party.
Taken to epic levels in Final Crisis: Superman Beyond with the Composite Character Captain Allen Adam, A.K.A. the Quantum Superman of Earth 4, who is a meshing of the Good Doctor with the original Captain Atom. Even when on heavy drugs to keep his quantum senses in check he's capable of feats of extreme mental prowess, and after his drugs fade off he attains a state of nigh-omniscience and arranges the activation of the Cosmic Armor that saves all of reality from an Eldritch Abomination that eats stories. Only The Spock can stay calm and babble about the organic nature of The Multiverse while the Ultimate Evil emerges.
Prowl in the original Transformers series, more so in the comic (which is why it's here). Shockwave, too, though he's a villain.
Skalman in Bamse is almost an example, but he is usually even less likely to suggest a course of action that seems unethical - in fact, it is sometimes shown that being logical and thinking things through allows him to do the opposite.
Linus often performs this function for Charlie Brown in Peanuts. Likewise Schroeder to Lucy (and occasionally Charlie).
The Treen of Dan Dare are completely emotionless and driven by logic. Sondar, a good Treen, is capable of feeling emotion, but still approaches problems logically.
Reservoir Dogs - Mr Pink is the most logical of all the crew, especially when he acts as the foil to Mr White. The first scene shows him refusing to throw in money to tip the waitress, giving his reason as to why. He is later distrustful of everyone, and disgusted that Mr White gave Mr Orange his real name and hometown, considering anybody, even the dying Mr Orange, could be the rat. Not only is he right about everything, but he is the only character to survive the film. Subverted in that Mr. Pink is anything but cool—he's both logical and wildly neurotic.
Dr. Egon Spengler from Ghostbusters. In the sequel, he says that his parents did not believe in toys and he seems to be nigh-immune to the mood slime which makes the two most "positive" Ghostbusters try to kill each other. In the commentary for the original film, Harold Ramis notes that he deliberately played Egon as a Spock-like character. He also notes that there is only one scene where he actually shows emotion, when Walter Peck tries to have the Ghostbusters arrested for an explosion that he caused himself.
Juror #4 (the stockbroker with wire rim glasses) from 12 Angry Men.
Sunshine. When the crew out to save the sun (and the world) are down to five members and their oxygen supply is cut, they realize they only have enough oxygen for four people to make a return trip. Michelle Yeoh doesn't bat an eyelid in suggesting they murder one of their own (whom happened to be already wracked with guilt for a mistake that led them into this predicament in the first place) instead of all five of them dying from lack of oxygen, and it didn't take much convincing for two other crew members to accept the idea.
Although in terms of Iron Man and Captain America alone, these two show a different dynamic with Steve being The Spock. He is the tactician of the team and less of an ideological hero than he was in his own movie, while Tony is clearly dismayed at how calm he is following Coulson's death and has a moment where he wins Bruce over through sheer compassion.
Death in the Discworld series. Is attempting to understand the human race, but is finding it...difficult. At times he seems to know a bit more about human nature than he lets on though, so maybe it's just an act.
Voort "Piggy" saBinring from Wraith Squadron. He's a Pig Man from a species more known for violent aggression and stupidity, but brain tampering made him into a Genius Bruiser, Good with Numbers and tending to be logical and calm. We see a bestial side exactly once, and it takes getting gut-shot to bring it out. Sort of similar to Vulcans, who practice emotional suppression precisely because their emotions are so violent and difficult to control compared with other species.
Michael Valentine Smith from Robert A. Heinlein's Stranger in a Strange Land. He is capable of feeling overwhelming emotion, but when he does, he simply slips into a coma until he's found a way to logically respond to the situation. As a human raised on Mars, it takes him quite a while to understand laughter, grief, fear and loss - his only emotions at the start of the book are curiosity and love.
C.S Forester is fond of this. The captains in The Good Shepherd and The Captain from Connecticut were very extreme Spocks. Horatio Hornblower was a slightly more mild version but still something of a Spock.
Data from Star Trek: The Next Generation. Subverted, though, in that Data is actually aspiring to become more human, and makes significant strides toward that throughout the show and its films. Still, as Spock himself put it:
Spock: Fascinating. You have an efficient intellect, superior physical skills, no emotional impediments... there are Vulcans who aspire all their lives to achieve what you've been given, by design.
Dax originally had shades of this in Star Trek: Deep Space Nine (this is lampshaded in a time travel episode where she unknowingly parrots Spock's statistical readings), but grew organically into a party girl and textbook case of Immortal Immaturity. The acerbic security chief, Odo, is much closer to this.
Seven of Nine from Star Trek: Voyager, though she tries to overcome this in later parts of the show.
Strangely enough, for all the jokes of his lack of intelligence, this is usually the role Jayne Cobb plays in Firefly. Ruthlessly pragmatic, Jayne will always be the first one to suggest that the smartest course of action when in a bind would be to betray this person or abandon that cause. Rarely is his logic on these matters ever called into question, but no one else is willing to be that cut-throat.
Temperance "Bones" Brennan from Bones. This becomes funny given that another character called "Bones" from Star Trek is the original McCoy. Also, Zack Addy.
Teal'c of Stargate SG-1 is sometimes The Spock, in that he has the same unflappability and (usually) rational thinking, though he's more The Stoic than a person who actually doesn't experience emotion.
Ficus in Quark is a parody of The Spock taken to extremes. He's a sentient plant and has absolutely no emotions, taking everything logically and speaking only in Spock Speak.
Aeryn Sun of Farscape is a minor tactical genius and never loses her cool...perhaps to her detriment, as the path of her relationship with Crichton is rocky and convoluted. She becomes more The Kirk as she goes along.
Sikozu and, after his Heel-Face Turn, Scorpius are perhaps the purest Spock members of the crew,having a tendency to be coldly pragmatic about problems.
Rygel is a slight subversion, as he has a tendency to on occasion claim his actions are for the best of the crew as a whole, but more often than not he's just being a greedy self-interested Jerk Ass. He has his moments, however.
Sheldon in The Big Bang Theory. Lampshaded, as Sheldon considers his circle of friends one doctor short of a landing party, and has proclaimed himself the Spock. Sheldon might not be a full-fledged example of this trope, though. He's too self-centered to be detached from the situation, so to speak - but Leonard's mother, Beverly, fits this trope very well.
Rupert Giles in Buffy the Vampire Slayer. Subverted, as he is often flustered or baffled, but forces himself to adhere to the cold-headed British persona he was raised to believe was most proper for any occasion.
Jor-El in Smallville. He often cooks up options for Clark to beat the bad guy that involve sacrificing his friends. He is however, prone to getting royally pissed at Clark for refusing to listen to him.
Robert McCall, The Equalizer, is very much this but has tempered it some on his own by choosing to stick to ideals he had put away for the greater good when he was part of The Agency. He always prefers logic, rationalism, and hard facts to intuition, and never lets his emotions rule instead of his head, always calculating and manipulating the odds, and willing to pay the cost himself to save someone's life.
Also, Control from the same series, the only person in the series to out-Spock McCall and to whom McCall could look like a McCoy next to.
Alchemical Exalted have a trait called "Clarity" that tracks how much of this personality they accumulate; as they grow in Clarity, they become increasingly focused on pragmatic and efficient solutions and will place lower priority on compassionate activity, at the cost of becoming increasingly less emotional and having difficulty relating to other people (they also have some Charms that can harness Clarity to grant increased cognitive ability). Clarity is reduced by prolonged meaningful interaction with normal human beings (or, in the case of some Alchemicals, immersion in memories from their past lives).
Among the Primarchs of Warhammer 40,000, there was Ferrus Manus on the Imperial side and Perturabo among the traitors. Both tended to be the most dispassionate and calculating Primarchs, and their Legions followed suit - although Perturabo's claim is a little weakened by his towering resentment and susceptibility to brief spasms of uncontrollable fury.
A rare main character example: Brutus from Shakespeare's Julius Caesar, with either Cassius or Antony as The McCoy. A quiet, (literally) stoic, cool-headed intellectual who is a friend of Caesar's but is willing to do him in since, to quote the Trope Namer, the needs of the many outweigh the needs of the few. ("Not that I loved Caesar less, but that I loved Rome more!")
In Shakespeare's Troilus and Cressida: Ulysses on the Greek side, with Agamemnon as The Kirk and Ajax as a fairly thickheaded variation on The McCoy. On the Trojan side, Hector fills this role, to Priam as Kirk and Troilus as The McCoy
Mordin Solus, though Hidden Depths reveal that he's way more compassionate than he lets on and feels incredible regret for his Well-Intentioned Extremist moment (though he feels it's still the right choice).
The Ruthless backgroundShepard also qualifies, doing whatever it takes to get the job done, no matter how horrific, and usually in the middle of the action himself—the military's go-to guy for the most vital and most morally compromising missions. The Renegade morality path also comes off as this in all the major decisions and in many of the conversations (with the rest of the time consisting of being a hardass, and in some instances in the first game, a bit of a xenophobe as well)
Squall of Final Fantasy VIII is a rare main character version. Raised as a mercenary, he's cynical about how the world works and tries to act solely according to duty rather than emotion. For the first portion of the game he seems like a lone Spock in a team of McCoys, but much of his Character Development is him learning how to healthily feel and express his emotions rather than acting like an automaton until he blows a gasket over, say, the execution of someone he knew from childhood.
Jade Curtiss of Tales of the Abyss, despite his Stepford Smiler outward persona, fits this trope when it comes down to actual decision-making. It's a Crowning Moment of Heartwarming the one time that he does admit he'd rather not do the most coldly logical thing, which would be to ask Luke to sacrifice himself rather than Asch.
Jusqua of Final Fantasy: The 4 Heroes of Light thinks that he's this. When Horne is petrified, he doesn't go with Brandt and Yunita because they can find a way to lift the curse. Later, he faces up to the fact that he was actually being immature by snarking at them when he himself was unwilling to try anything he thought he'd fail at.
Dangan Ronpa has Kirigiri, who is coldly aloof, calculating, and emotionless. Celes is one as well, what with her saying they should abandon all attachment to the outside world and just accept their fate living in Hope's Peak. Togami is a milder, crueler version.
Vaarsuvius from The Order of the Sticktries to be this in order to become more effective and overcome a great obstacle, but seeing as how the elf is anything but emotionless and is in fact haunted by the memory of a great failure, the result is disastrous.
The AI Delta from Red vs. Blue is logic personified. Literally. Ironically, Delta is probably the most humane of the various Freelancer AIs, despite his noticeably incomplete understanding of human nature.
In My Little Pony: Friendship Is Magic, Twilight Sparkle takes this role in the first two episodes, not caring about "pointless" things like friends or fun. She ends up coming around at the end of the second episode, and the rest of the season documents her learning about just how kick ass friends are. She occasionally lapses into this when academic subjects are mentioned, especially magic. She has a very logical, scientific view of the world and though her excellent education means that she's often right, it does leave her rather inflexible. Her tendency to dismiss information that she finds illogical in particular has been called on more than once.
McTavish: Are you sure you've never served as a science officer?
Dr. Herman Kahn. His works in the 1950s on nuclear war examine the aftermath in extremely dry terms. He was considered a sort of monster by some to actually argue that, while extremely horrible, a nuclear holocaust would not be the end of humanity.
By his own admission, Israel's current Defense Minister and former Prime Minister Ehud Barak. He and everyone around him quite frankly states that Barak's mind is like a steel trap: remorselessly methodical and logical, remembering everything, and expecting everyone to be just as logical as he. Barak, however, does have the humanity to admit that this is something of a double-edged sword: his logical method blinded him to the emotions of others, particularly the Palestinian and Syrian diplomats he tried to negotiate with at Camp David in 2000. This insensitivity—again by his own admission—probably cost him the deal of the century.
The leopard, in comparison to other big cats. Lacking the teamwork of lions or the raw strength of tigers, leopards rely on a combination of cunning, versatility, and calculated risk to survive. They are opportunistic hunters that carefully judge their prey before attacking, and actively avoid confrontation with stronger competing species.