The Foil for a hero of the closely allied variety.
In general, the Lancer will often (though not always) form one half of a Red Oni, Blue Oni contrasting duo with the Hero. Specific ways this can manifest include:
If The Hero works more from a state of passion when resolving problems his Lancer tends to work harder, and in some but not all cases tends to be less popular. The Lancer in this context tends to think his way through challenges.
Similarly, if there's only one stealthy guy on the team, The Lancer is typically that guy. Sneaky Lancers may fill the Fragile Speedster (or, if lucky, Lightning Bruiser) role, relying on agility and guile rather than brute force.
The Lancer may be a Badass Normal in a superpowered team. On the other hand, if most of the team is more specialised or noncombatant, The Lancer will often be the most skilled and dangerous fighter on the team. (not necessarily the strongest, that's reserved for The Big Guy)
Iceman was the Lancer of the original five, with an attitude bigger than Connecticut and a constant bone to pick with The Hero, Cyclops.
A solid case could be made for Angel as the Lancer of the original five, actually, right down to being a rival for the affections of The Chick. Unlike Iceman, Angel has actually done the leadership thing on occasion.
Wolverine (and Rogue, under similar circumstances years later) panicked and tried to refuse when ordered into a leadership position (despite proving rather competent in same). Nobody has had the nerve to try that stunt on Iceman. In fact, Wolverine was even slated to lead Alpha Flight when he ran away to join the X-Men instead (he was also in love with the team's eventual leader's red-headed wife.) Then again Logan has taken leadership positions since.
Skywise from ElfQuest has a great number of complementary traits with his buddy and heroic leader Cutter. A drinker and womanizer, he is more lively and easygoing than Cutter, who naturally bears a keen sense of responsibility for the tribe. Skywise is a studious, curious dreamer, while Cutter prefers to live in the here and now.
A more likely interpretation is that among the Wolfriders, Strongbow is the lancer and Skywise is the smart guy. Unlike Skywise, Strongbow makes a habit out of disagreeing with Cutter a lot. In the later series, e.g. Shards, Rayek becomes the lancer.
For the current New Avengers line-up Hawkeye/Ronin seems to be The lancer for Captain America (Bucky).
Actually, it's the other way around.
In Young Avengers, Kate/Hawkeye is the Lancer to Eli/Patriot. They butt heads near-constantly (though that might just be them flirting), but Kate genuinely believes Eli is the only one who can actually lead the team.
Donald Duck fills this role nicely for Scrooge in the Don RosaUncle Scrooge comics, serving as a practical, pessimistic Foil for the overly-eager treasure hunter, ever ready with either a complaint or a snarky observation.
In Sin City, Dwight has had two lancers over the course of the series. In the story A Dame To Kill For, he employs the help of Sociopathic Hero, Marv. Later in the same story and subsequent stories after that, his lancer is Action Girl, Miho. Both lancers are extremely loyal to Dwight but are also far more violent, causing him to be unsettled more than once by their brutality.
In Runaways Nico was originally this to Alex. These days Gert, Victor, and Chase have all been known to alternate the role amongst themselves.
Chaos from the Tamers Forever Series. An interesting example, as he is a split personality of The Hero Takato. He is essentially Takato's equal and opposite. They posses the same skills, but Chaos is ruthless where Takato is compassionate.
Jack Sparrow, Will Turner and Elizabeth Swan in Pirates of the Caribbean seem to take turns playing The Hero and The Lancer. They switch sides and betray each other so frequently it can be hard to keep track of who's on which "team" and who's just out for their own ends. The only person who seems to consistently fulfill this role is Joshamee Gibbs, as the Lancer to Jack Sparrow.
The WWII biopic Patton depicts a "lancer reversal" between American generals George S. Patton and Omar Bradley. At first Bradley is Patton's lancer when Patton is made commander of the II Corps in North Africa and Bradley his deputy. Then after the invasion of Sicily, Patton is reprimanded for slapping a shellshocked soldier and Bradley is promoted over him. Bradley commands the U.S. 1st Army during the D-Day invasion of Normandy. Meanwhile Patton was used to decoy the Germans into thinking the invasion would be at Calais. Patton becomes Bradley's lancer when he gives Patton command of the Third Army to implement "Operation Cobra", the Allied plan to breakout of the Normandy beachhead.
Snowman is this to The Bandit, hauling the all important cargo and occasionally saving the Bandit when the cops decide to get serious.
Arthur, the Point-Man, is the sensible and cool-head Lancer to the dark and troubled Cobb in Inception.
Iron Man to Cap in The Avengers. Out of the whole Dysfunction Junction, they have the most personality conflict for any number of reasons (idealist vs. cynic, reluctant soldier vs. ex-weapons maker), but still end up working well together. Hawkeye, by contrast, is the Sixth Ranger who only really opens up around Black Widow.
In Mr Saturday Night, Stan had the makings and ambitions to be a comedian but his brother Buddy Young had the guts to go and perform, so Stan becomes the more grounded manager to egotistical Buddy.
In The Princess Bride, Inigo Montoya fulfills this role rather well to the Man In Black/Westley. At first he is arrogant and apprehensive of Westley's skills, but eventually goes on to be a faithful right-hand man and gains a deep-set respect for his skills to the extent that he believes that Westley is capable of anything (which essentially turns out to be the case).
In Seven Psychopaths, Billy fits the role of the Lancer to Marty. Marty and Billy are almost perfect foils for eachother, since they have different opinions on Marty's alcoholism, Marty's girlfriends, and lifestyle. Marty has the Badass power (Billy says that Marty is the "best writer of his generation"), and Billy dies for the cause.
Orestes: Pylades acts in this capacity to his friend Orestes, in Euripides' play.
Romeo and Juliet: Mercutio is Romeo's right-hand man and best friend. He even goes so far as fighting and losing a duel in his name.
Casio, in Shakespeare's Othello. Casio is Othello's faithful lieutenant. He is a good friend to his general, but has a weakness for drink and women.
Tower of God: Koon Agero Agnis, the brains, and in a literal sense Rak Wraithraiser, the brawn (and The Heart), choose to climb the Tower with Baam, one to find his lost self, the other because Baam happens to own a really Cool Sword he wants to compete against.
Humorously enough, the author of Cwen's Quest was unaware of this particular trope when he named his character Riddly Lancer. Fits the bill too, well actually only up to the third sentence of the description that is... but Cwen is definitely the leader between the two characters.
In Sluggy Freelance, while Torg usually takes center stage and, in his "unique" way, come up with a solution to the big problems, Riff is usually there as well, ready to solve things his own way. Which usually involves laser weapons, giant robots, and blowing stuff up.
Kobe Bryant to Shaquille O'Neal from their generation of the LA Lakers. The seasoned Shaq was The Big Guy and the captain, while the rambunctious Kobe was the rookie. Their relationship was frequently punctuated by feuds played out in the press. Now that Shaq retired, Kobe's become the face of the Lakers, with Pau Gasol and Andrew Bynum sharing the role of The Lancer.
When they were both on the Chicago Bulls, Scottie Pippen was seen as a lancer for Michael Jordan.
Russell Westbrook, the cocky Red Oni, to Kevin Durant, the collected Blue Oni. Unlike Kobe and Shaq, Durant and Westbrook are Heterosexual Life-Partners.
Dwyane Wade to LeBron James in the Miami Heat. James is the theatric champion on the court; Wade is the leader and the face of the team.
The Chicago Blackhawks are led by young superstars Jonathan Toews, nicknamed "Captain Serious", and Patrick Kane, who does not exactly carry the same reputation.
Alex Rodriguez to Derek Jeter after he joined the New York Yankees.
Bobby Bonilla to Barry Bonds, during the competitive 90s Pittsburgh Pirates, before their downward spiral to becoming one of the worst teams in over a decade. Bobby Bonilla would go with coach Jim Leland and win a world series with him on the Florida Marlins. Barry Bonds would go to the San Francisco Giants and put up great career numbers. But he would never win a World Series and then you have the steroids accusations.
The Who is an interesting example. Pete Townsend was the band's artistic leader, but was also violent and dealt with depression. He wasn't the band's frontman, though. He left that up to "second-in-command", Roger Daltrey who was more reserved in comparison and often served as the more mature member of the group.
in West's own crew, GOOD Music, Pusha T is the Lancer to West's hero.
Ace Pilot Staff Sergeant Nils Katajainen (36 kills) to Captain Hans Wind (78 kills) in the Finnish Air Force in WWII. They usually flew as a pair. Both were awarded the Mannerheim Cross, roughly the Finnish equivalent of Victoria Cross or Congressional Medal of Honor.
Michael Vick to Donovan McNabb in the Philadelphia Eagles. McNabb was the calm, collected quarterback leader with a passion for the sport, while Vick was the "bad-boy" second-in-command who was in legal troubles, especially with the dog-fighting.
During his last three seasons at Manchester United when he established himself as one of the best players in the world Wayne Rooney was often his lancer.
Alexander Hamilton. Hamilton served as George Washington's Number Two during the Revolutionary War and Washington's time as president. While Washington was stoic, deliberative, and against bare knuckle politics, Hamilton was brash, quick witted, and famously one of the most vicious political fighters of his generation. This proved useful to Washington, who relied on Hamilton's intricate thinking and writing skills to serve as his Army chief of staff and most trusted secretary, much to the annoyance of others on Washington's staff.
Shuuichi Ikeda to Toru Furuya; they voiced for the main antagonist and the main protagonist of Mobile Suit Gundam respectively.
Michael Wittmann, one of the most successful armored commanders of all time, had Bobby Woll as his gunner. They were so in tune that, eventually, all Wittmann had to do was give Woll a direction where enemy tanks could be found and let Woll do the rest. Luckily for Woll, he wasn't with Wittmann when the latter's luck ran out in August, 1944.
General Thomas "Stonewall" Jackson, to General Robert E. Lee once Lee earned command of the Virginian forces in 1862. When Jackson was killed, accidentally shot by his own men at Chancellorsville, Lee said "He had lost his left arm, but I have lost my right."
On the Union side, William Tecumseh Sherman was The Lancer to Ulysses S. Grant. "Grant stood by me when I was crazy, and I stood by him when he was drunk. Now, we stand by each other always." Grant entrusted Sherman to handle the Western theater of the Civil War while Grant assumed command in the East to fight Lee. It worked.