24: Tony Almeida fits almost every part of this description in relation to Jack Bauer. This is an interesting case in that Tony initially seems like a calmer, saner, more soft-spoken counterpart to Rabid Cop Jack. But placed under enough strain (as exemplified by how they each handle the loss and/or potential loss of their wives), Tony can actually be far more emotional, reckless, self-destructive and vengeful than Jack.
Agents of SHIELD: Melinda May. She's recruited to be Phil Coulson's second-in-command, and her Ice Queen demeanor contrasts with Coulson's friendlier one.
Several characters have Lancer qualities. Doyle was the initial one, but for most of the series, Wesley fulfilled this role as well as The Smart Guy. Gunn also filled this role for a time late season 3 and early season 4, but was usually The Big Guy. Connor fit for much of season 4. Easily the best fit that doesn't overlap with any other roles is Spike in Season 5.
Wesley, true to the trope, was the ruthless member of the team, as well as the sorcerer of the bunch, also betrayed the gang to save Connor, stabbed Gunn and shot Knox during his Roaring Rampage of Revenge, and, being the less stable of the group due to Fred's death, was expected by Cyvus Vail to make a move for Angel's position. Aware of it, Wesley pretended to be planning to betray Angel in order to get close to Vail and kill him. As the trope goes, Wesley was killed by Vail instead.
Cordelia was Interim Lancer between Doyle's death and Wesley's integration into the main cast, and was always Angel's real foil, despite being The Chick throughout.
Garibaldi is more aggressive than Captain Sheridan, prone to independent action, more of a covert operative, and does indeed get brainwashed at one point.
Ivanova is the pessimistic - sorry, make that Realisticdry-witted executive officer. She's remarkably tenacious, hammy when angered, and a spy master on par with Garibaldi at his best. Her fiery personality particularly contrasts with the original commander, Sinclair.
Buffy the Vampire Slayer: Willow and Faith can act Lancer-y. Oh, and also Xander, despite also being the Butt Monkey. In addition, Cordelia was this in the earlier seasons and when she left to be on the show Angel was replaced by Spike.
CSI: Miami: Ryan Wolfe has qualities of this. However, Eric Delko is a better example of the Lancer to Horatio's hero. Womanizer, leader, outgoing, and willing to take initiative on his own to further the team. Add to that he's completely loyal to Horatio.
In the first serial, Ian prevented The Doctor from bashing a caveman's skull in. This has been one of the reasons The Doctor keeps companions, to keep his humanity in check and prevent him from slipping into darkness. So to an extent, they're all Lancers. Episodes like "The Waters of Mars" and "The Family of Blood" in which his companions couldn't control him, show this very well.
Captain Jack Harkness filled this role during his very short tenure.
Doctor Who usually has a small ensemble, but groups exceeding three members would occasionally contain Lancers; for example Rory Williams and Ian Chesterton, both more 'classical' heroes in contrast to the Anti-Heroic Doctor, and Tegan Jovanka, an unwilling passenger in the TARDIS who eventually left on her own accord. The thing about the Fifth Doctor adventures is that his companions hardly ever seemed to tolerate each other (Turlough, Tegan, Adric), apart from Nyssa.
Donna Noble, one of the most popular companions of the Tenth Doctor, shares a similar background and personality with Tegan.
Tegan retained her old personality in her early Big Finish appearances. This was a bit too much for her opening story. She always was a moaner but this was taken so far it was almost a parody; The Gathering suggests that traveling the TARDIS ruined her life and gave her brain cancer. She calms down a bit in Heroes of Sontar and Emerald Tiger and provides some of the more amusing moments in later trilogies. It's almost hard to reconcile this sunny, smiling TARDIS team with the same ones who glowered throughout "The King's Daemons"....
Joe Ford: Tegan is moaning because she doesn’t like the idea of Turlough traveling with them (and no its isn’t because she wants the Doctor all to herself, pipe down all you shippers at the back!) ...It's true that she is at least very honest, there is nothing that she would say behind Turlough’s back that she wouldn’t say to his face. More’s the pity: because now we have to listen to it twice. Then once all the bile is extracted from her gut she turns round and tells Turlough that he should stay! You know when hackneyed male figures in fiction stick their hands on their hips and cry ‘women!’ at something exasperating? I think that should be adopted here but the word changed to ‘Tegan!’
Gobo, the reluctant leader of the Five-Man Band, had two extremely opposite Lancers — his best friend Wembley and Wembley's constant rival Red.
Although they get considerably less screen time, the Minstrels are a clear cut Five-Man Band, and it's obvious that Murray acts as the Lancer to Cantus.
Giant Saver: It is a sentai show, but Thunder Guard Xu Weizhen takes the cake on the whole "rival and foil to The Leader" part of the trope. He is constantly a jerkass toward Dakong, the Fire Guard, and his cynical, money centered world view couldn't be a better contrast to Dakong's Wide Eyed Idealism.
Happy Days: The Fonz fits the definition so well it's almost scary. What made the show a hit was the contrast between super-cool ladies' man and high school dropout Fonzie, and the gutsy, strait-laced, all-American Richie Cunningham. As shown in a flashback, the two didn't initially see eye-to-eye, with The Fonz being in a gang. However, he is impressed that Richie had the courage to stand up to him, and quit the gang he was in because of Richie's speculation that he was cool enough without a gang jacket to show for it; afterwards, the two became close friends and, when Richie left to pursue his dream of being a screenwriter, his final words were to Fonzie, who he dubbed a brother and a protector.
Hotel Babylon: In this British series, the concierge Tony is a seasoned pro at the hotel business and very well-respected among his colleagues. When a general manager strays, he's usually one of the first to stand up for the good of the hotel or its staff and let a manager know he's going in the wrong direction.
Foreman is the subordinate most likely to oppose House on any given matter, and in later seasons has become the de factoNumber Two of the team (having stepped into the position of leadership on three separate occasions). Though his characterization has emphasized a fundamental similarity between himself and House, there are important (perhaps deliberate) differences — Foreman is stuffier than House, presenting an air of consummate professionalism in contrast to House's slip-shod and free-wheeling style of management.
House of Anubis- Patricia Williamson, being the Anti-Hero to Nina's Hero, and being the most independent member of Sibuna. She also fits this role in the third season Sibuna as well.
Inspector Lynley: DS Barbara Havers to the Inspector; she challenges him at every turn and is the first to tell him when she thinks he's being an idiot. She would also follow him into fire if he asked her to, and anyone who challenges him will find themselves on the wrong end of her fury.
With Stabler's departure, Benson has been moved up to The Hero and new addition Nick Amaro takes the role of Lancer.
Leverage: Sophie most cleanly fits this role, often serving as a foil to Nate in the planning process, though the rest of the crew also occasionally fits into it at times. Eliot in some ways fits this role, as the opposite of Nate, preferring to fight his way out of situations then Nate's Chessmaster style. Parker in some ways fits into this as the only true thief among them, as well as being a skilled planner when it comes to heists. Hardison also is trying to develop the skills to run his own crew as well.
Lost: Locke and Sayid alternated on this role towards Jack until Locke leaves the group in season 4. Ana Lucia also served as this for a brief time in season 2, before she was killed by Michael.
When Jack and five other survivors leave the island in the season 4 finale, Sawyer becomes The Leader with Juliet as The Lancer. In fact, Juliet's "I have your back" to Sawyer and acting like she meant it became a pretty important part of season 5 that leads to the two of them getting together.
And talking about Jack's group in season 5; Sayid, taking jobs for Ben as an assassin, becomes The Big Guy, which moved Kate to The Lancer of the group.
Season 6 had the people on the island divided in two camps: Jacob's group and the Man in Black's group. Ben served this purpose to Ilana/Jacob's group as of "Dr. Linus" (Ilana didn't really trust him earlier than that), while Claire and Sayid alternate to serve the Man in Black as this.
Mash: Trapper John (seasons 1-3) and B.J. (seasons 4-11) serve as this to Hawkeye.
And even Abby, usually The Heart, will go that way.
Noah's Arc: Ricky, as a very clear foil to Noah. While Noah is very moral, relationship-oriented, romantic, and holds the group together, Ricky is much more pragmatic, highly promiscuous, sexually detached, and more than willing to abandon the group over a one-night stand. The stark contrast between them is lampshaded by both of them at different points in the series. Also, Ricky, Noah and Wade are involved in a love triangle highlighted in the movie.
The Office (US): Jim is briefly promoted to co-manager and tries to be a lancer to Michael, tempering his need for fun with a need for work.
Once Upon a Time: In season two, Red is this to Charming's Hero, especially with Snow stuck in the Enchanted Forest.
Revolution: Miles Matheson is the opposite of Charlie Matheson. He's older, smarter, more cynical and more experienced compared to her.
Roswell: Michael frequently plays the classic Lancer to Max's Hero, as both lifelong best friend and, with his loner persona and occasional need for anger management, as his foil. Michael even briefly assumes the role of "king"/leader after Max's "death".
Plus, he stops himself from being clinically dead for his friend.
Smallville: Clark has had a few Lancers in the ten seasons of the show. In the first few seasons it's Lex, his best friend and counterpart who is often portrayed as somehow being both Clark's polar opposite and kindred spirit. Once Lex starts to go evil, Oliver takes over the role.
Star Trek: Spock is all over this trope. Indeed, he is by all counts equally as competent as Kirk. But where Kirk will go with guts and daring, Spock will always be cool, calculating, and logical. And yes, when the chips are down, Spock is ready at a moments notice to die for the cause (as seen in Star Trek II). Also, in cases where Spock devoted most of his screen time as The Smart Guy, Dr. McCoy picked up the role.
Star Trek: Deep Space Nine: Kira is Sisko's lancer in every aspect of the trope. If it were up to her, there wouldn't be any Starfleet personel on the station and she would be in command. In early seasons, she's yelling at Sisko every other episode and often completely ignores his orders when she believes she can handle things better. She grows to trust him in later seasons, but she always remains the person who challenges the captain's orders the most—somewhat unusually, considering that, as the Emissary of the Prophets, he's the equivalent of a saint or prophet of her religion.
Riker was intended to perform the action roles that Picard (as The Captain) really should not be doing. Especially in season 1, he was a Kirk clone. After Growing the Beard, his role on the bridge was to anticipate Picard's orders and issue them so that the captain wouldn't have to.
Contrasts with Picard in Riker being a womanizer (Picard is not very successful with women) and somewhat impulsive (Picard is fairly cautious). Riker also fits the role in easily being the more gregarious and social (he plays in the ship's jazz group and runs the senior officers' poker games) while Picard is the more private and prefers solitary pursuits such as reading and archeology. Oddly enough, while Riker is obviously supposed to be the more physical (being younger and larger) compared to Picard's more intellectual, Picard tends to be the one who ends up in the most physical confrontations over the span of the series and movies.
Star Trek: Voyager: While he's not the first officer, Tuvok fulfills this role for Captain Janeway. He's calm and rational while Captain Janeway is usually very emotional. In later years, Seven of Nine plays the role.
Top Gear: Richard Hammond to Jeremy Clarkson. Hammond is a head shorter, a decade younger, and physically tough but emotionally fragile compared to Clarkson, yet they are inseparably bonded by a mutual love of fast, powerful cars. The third presenter, James May, acts a bit like a Lancer to Clarkson but they are more of an Odd Couple.
Torchwood: Owen falls under this fairly well, at least in series 1. Owen is The Lancer to Gwen while Jack is away. When Jack returns, Gwen becomes his Lancer.
The Walking Dead: Shane Walsh for the first two seasons. Daryl Dixon from season 3 onwards. Until season 3, Andrea is this to Lori for the women.
The West Wing: Leo is a dead-on Lancer of the Blue Oni type to Bartlet's hero. Being The Lancer is also Josh's calling in life, according to Bartlet: "You know the difference between you and me? I want to be the guy. You want to be the guy the guy counts on." In the later seasons other characters lampshade the fact that Santos, to Josh, is his opportunity to be what Leo was to Bartlet.