American Gods: Shadow's old cellmate, Low Key Lyesmith. Keep in mind that Norse gods play a huge role in the book, and he is actually Loki Lie-smith.
As a series with a fifty-four book run, not to mention prequels and Megamorphs books, Animorphs had quite a few of these. There weren't as many as you'd expect, due to the latter half of the series being ghostwritten, but there were still plenty of meaty examples for discerning reading to enjoy.
Visser Four, first mentioned in an aside in The Decision, becomes the main villain of Megamorphs #03: Elfangor's Secret.
In The Arrival one of the four Andalites is presented as a mysterious nobody. He's eventually revealed to be The Man Behind the Man.
The ultimate Chekhov's Gunman for Animorphs, though, has got to be Crayak. The Big Red Eye first appeared to Jake as a vision at the end of The Capture. For the next twenty books this goes unmentioned, and then along comes The Attack and the big reveal.
In the beginning of The Departure, it's mentioned that a leopard escaped from someone who keeps wild animals. The leopard ends up playing a huge role in the story.
Lampshaded in the first Artemis Fowl book with the introduction of Chix Verbil, whom the narration says will become important in sequels, but "for now his only function is to press a button to activate the time-stop".
Played straight with Turnball Root, appeared in a short story and reappeared in the Atlantis Complex.
That sprite who told Turnball how to re-acquire some magic? That's the first supernatural being who ever appeared on-screen: the fairy who loaned Artemis a copy of her Book way at the beginning of the series. Given that she hadn't appeared since the first chapter of the first book, and this was the seventh, this probably is also a Brick Joke.
Atlas Shrugged: Who is John Galt? The unnamed minor character standing in plain sight for six hundred pages, rather than a rhetorical question.
In Lawrence Block's The Burglar in the Closet Frankie mentions that the late Crystal Sheldrake had three (concurrent) boyfriends, one of whom is a lawyer. Since she does an Ed McMahon impersonation when mentioning this, Bernie decides that said boyfriend must be named John and after he and Jillian realize the difficulty of locating him without a surname they concentrate on the other two boyfriends as suspects. It's eventually revealed that the guilty party is Crystal's ex-husband's attorney, Carson Verrill.
Cal Leandros': Madhouse has Seraglio, Robin's housekeeper, who seems to get introduced for the sole purpose of disapproving of Robin's wayward ways and making pancakes. At the end oft the book, though, she turns out to be the one behind the assassination attempts on Robin.
A lot of gods/mythological beings tend to be mentioned much earlier than their physical debut. Below are just the notable examples:
The Twelve Olympians are mentioned by name in The Lightning Thief, but they are introduced sequentially, with only Dionysus, Ares, Poseidon, and Zeus appearing in the book. Demeter doesn't make an appearance until The Last Olympian, four books later.
Aeolus is mentioned one book before his debut in The Lost Hero.
Nemesis and Hecate are similarly second series deities who are mentioned in the first (they are said to be serving the Titans).
Tartarus has been mentioned now and then, but only becomes important in The House of Hades. Justified since up until then, he is only considered a place by most (as the prison for the worst of the worst), so the revelation that he is a deity (and the father of the Giants, no less) comes as a genuine surprise for everyone.
Rhea gets a blink-and-you'll-miss-it mention in The Lightning Thief. She doesn't appear until The Hidden Oracle, two series and ten books later.
The little girl Percy spots sitting near the camp bonfire in The Lightning Thief is revealed to be Hestia, the titular character of The Last Olympian.
Circe's female staffer who serves Percy in The Sea of Monsters is reintroduced in The Son of Neptune as Hylla Ramírez-Arellano, the current Queen of the Amazons.
Will Solace has been mentioned since The Last Olympian, but it isn't until The Blood of Olympus that he plays a substantial role in the fight against the Romans. As of The Hidden Oracle, he is Nico's boyfriend.
Captive Prince: Guion's wife Loyse hangs in the background for most of the third book, ostensibly as a hostage to his good behaviour. It turns out that Laurent brought her to deliver the testimony that damns the Regent, as he expected Guion to perjure himself.
In The Caves of Steel, R. Sammy, a robotic courier, is the first "person" we meet in the book, but he seems more of a simple narrative device to provide an opportunity for exposition about the Earthmen's dislike of robots. Much later on, he is found destroyed, and it turns out he was a critical part of the book's murder plot - the one who sneaked the murder weapon into the area before the crime.
In The Chronicles of Ancient Darkness, The Walker, a crazy old man, is introduced in the 1st book, but it isn't till the sixth that we learn he is the 7th Soul Eater. He did give us some hints, though, like when he said that before he went insane, he was "a very wise man".
C. S. Lewis' The Chronicles of Narnia: The Silver Chair has a particularly devious one. Jill and Eustace are sent to look for the kidnapped King Caspian's son. Halfway through their journey to the place where the prince disappeared, they find a delightful young damsel escorted by a silent knight who doesn't show his face. If you haven't read the book, you have correctly guessed by now that the knight is the prince that they were looking for. However, the damsel is, in fact, the Big Bad that appears to the children to point the direction of a castle inhabited by giants for whom humans are refined cuisine delights. And the children never even suspect about the identity of the two strangers until the climax of the book.
In Craig Shaw Gardner's Cineverse Cycle, several minor characters from the first and second books were later revealed to be disguises used by a superhero called Captain Crusader, whose decoder rings could transport the wearer into and through the different worlds of the Cineverse whenever they said the code phrase.
Circleverse: In the Circle of Magic quartet's first book, Tris mentions to Niko that she has a cousin Aymery at Lightsbridge. In-universe, Aymery is as common a name as Steve, so when she hears it on the wind in Tris's Book she dismisses it as coincidence. Aymery himself arrives at the Winding Circle temple to visit a bit later acting as a spy/saboteur for the pirates who want to raid the place.
In the opening of Dorothy Gilman's The Clairvoyant Countess, Madame Karitska does a reading on Lt. Pruden and tells him that he will be married within fifteen months, to a woman with long, very pale blond hair and considerable psychic ability. When, later in the book, he is not able to talk straight upon meeting a woman with long, very pale blond hair and considerable psychic ability, he doesn't get it, but Madame Karitska smiles upon him and assures him that it is a very good thing.
Butcher also does this a bit with Codex Alera only the gunman turns out to be a "gunspecies". Tavi and Kitai go into the Wax Forest for a test and end up not only encountering the wax-spiders, but also an unusual new creature: the Vord queen. They're mentioned in passing during the first book but show up in each new one, getting more dangerous each time.
In Dangerous Spirits, Niki's father receives a vague reference in passing in Green Fairy. Turns out it's Konstantine.
From the Deryni novels: In what amounts to a cameo role, Bishop Henry Istelyn first appears as a previously-unnamed itinerant bishop who delivers a notice of excommunication to Kelson early in High Deryni. In the sequel The Bishop's Heir (set two years later), the loyal Istelyn is elected to the episcopal See of Meara and his fate becomes a major part of the book's plot.
The first Dinotopia book featured a minor character named Lee Crabb. He ends up becoming the antagonist in all the subsequent books (barring First Flight, which is a prequel).
In the very first book of the Dora Wilk Series, one of the magicals Dora saves from the Thief is a werewolf named Jędrzej. Come the final book, and he appears as an informant for her, claiming he's indebted, and by the end of the book he becomes the new alpha of Thorn.
The Dresden Files is very prone to this the main opponent of a book will almost definitely be someone who was introduced in an earlier book, quite possibly Grave Peril.
Turn Coat has another major one: The obnoxious little secretary wizard who tries to get Harry to sign for a folder he was getting off the record turns out to be (one of) the traitor(s) on the Council. And he was actually trying to get Harry to sign because he was using special ink for signatures to screw with the wizards' minds. Said character was actually introduced as far back as Summer Knight in passing.
Alongside this character was a woman who was described as old and holding a scepter. It is later revealed to be Anastasia Luccio, who is Captain of the Wardens, eventual lover of Harry Dresden, and brainwashed victim ordered to kill.
Molly Carpenter. Initially just one of Michael's many kids, she is first mentioned in passing in Grave Peril by Lea, Harry's fairy godmother, and briefly appeared in Death Masks as a background character. She becomes a major player in later books when she begins to manifest magical abilities and Harry takes her as an apprentice. At the end of Cold Days, she becomes the new Winter Lady.
Mab, Queen of Air and Darkness, gets name dropped by Harry as far back as the first book, Storm Front. She would end up becoming a major player in really short time.
A particularly impressive one from David Eddings. In The Ruby Knight, the second book of his Elenium trilogy, Zalasta is introduced in what is practically a throwaway scene; literally, he's seen for less than a page. He's not seen or mentioned again in the entire trilogy... and then he shows up in The Tamuli trilogy as one of the major characters, and ultimately turns out to be the closest thing to a Big Bad for both trilogies.
Also a lesser case introduced in the first book, an unnamed woman goes to learn magic from a Zemoch and becomes corrupted by Azash. She is then revealed in the Ruby Knight to be the sister of a noble, tormenting and sacrificing people to Azash and forcing Sparhawk and Sephrenia to break her power by destroying the idol of Azash she created.
A Fantasy Attraction has Aleksandra, a dragon who is collecting for her hoard. She is invited in, and you promptly forget about her. She shows up later just in time to incinerate a tribe of ogres.
The Four Feathers has Abou Fatma. He appears briefly in an early chapter, delivering General Gordon's letters to Jack Durrance, in a moment that merely establishes him as an Arab loyal to the British. He doesn't reappear until the very end, when he's revealed to be Harry Faversham's friend and helps him and Colonel Trench escape from prison. In the 2002 film he's given a much larger role.
To elaborate, his power is the ability to change his mass, from able to float to sinking through the ground. The sinking through the ground part turns out to be the only way to permanently destroy the gaiaphage, most likely digging it to the center of the earth, assuming his skeleton retains its mass after he dies.
Abigail Pent in Gideon The Ninth is a rare example of this being done posthumously. While it initially appears that she and Magnus were killed opportunistically, it's not until the final revelations of who the antagonist at Canaan House is that Cytherea reveals how Abigail nearly ruined her plans in three separate ways: her necromantic specialty in spirit talking had the potential to reveal the culprit behind other murders, her being a historian interested in piecing together Lyctoral history could have unveiled the secrets at Canaan House much earlier, and she had completed the challenge to gain the key to the Seventh House's Lyctoral study, which Cytherea had neglected to clear of traces of herself out of sentimentality.
Griffin's Daughter: Lady Amara mentions her predecessor as head of the elf mage circle had disappeared years ago. A book later, Amara's son - after being sold into slavery by humans - runs into Gran, an older elf maiden who turns out to be the aforementioned missing mage. (she was there of her own free will, as penance for an earlier tragedy.
In the Legends trilogy of the Guardians of Ga'Hoole series (books 9-11), a character named Theo is introduced midway through book 9, and he becomes the first blacksmith and creates the first battleclaws, but that's about it. However, later on, in book 13, it's revealed that Theo wrote the holy scriptures of the Middle Kingdom, and as such is directly responsible for the pacifist lifestyle of the owls of the Middle Kingdom, as well as being indirectly responsible for all the events involving the Striga, an escaped Dragon Owl who allies himself with Nyra and the Pure Ones in book 15.
The Harry Potter books did this to death, along with the other Chekhov's tropes. There were characters introduced in every book who became plot critical later, both in each individual book and across the series. There were even numerous instances of a character being revealed to be important more than once.
Sirius Black, mentioned in passing in the first chapter of the first book and revealed two years later to be the title character of the third book.
Mrs. Figg is introduced in the first book as Harry's babysitter, and we hear very little of her afterward. She is actually mentioned off-hand in book four as part of the 'old team' but it's very easy to forget about her. In book five, she's revealed to be a Squib that's been watching over Harry on Dumbledore's orders for the last fourteen years and is the sole witness to Harry's Hearing at the Ministry of Magic.
Aberforth Dumbledore first appears as the barman of the Hog's Head Tavern in Order of the Phoenix, although he isn't mentioned by name. Earlier, Dumbledore does mention him by name in Goblet of Fire.
The film makers wanted to leave Kreacher out of Order of the Phoenix entirely, but JKR told them that he would become very important later, as revealed in Deathly Hallows. (The Deathly Hallows film, however, cuts so much of Kreacher's role that his omission wouldn't have created any issues.)
Even Neville Longbottom could be considered one of these; even in book one, he has little to do with the overall plot, and he remains a Bit Character until Order of the Phoenix. In Deathly Hallows he cuts the head off of Nagini, the final Horcrux. He probably could have been introduced later like Luna was and it wouldn't have made much difference to the plot.
The savvyness of Potter fans in general meant many were delighted to realize, on rereading, that "that awful boy" Petunia refers to in Phoenix who taught Lily about Dementors is not James, as is assumed by Harry and the reader, but Snape.
Another living Chekhov's Gun comes in the first chapter of Goblet of Fire, which seems like an unnecessary bit of exposition regarding Voldemort's ultimately-unimportant Muggle father and the fact that Wormtail has found Voldemort, in which the two of them kill a Muggle. However, this is also the first appearance of Voldemort's snake Nagini, who later takes on a more important role. Each of the next three books makes this chapter more clear. In OotP, Voldemort sends Nagini to attack Arthur Weasley — and Harry, who has been having visions in which he sees what Voldemort sees, is somehow able to see the scene from Nagini's perspective. The chapter itself is explained in HBP, when we learn that a murder is required to create a Horcrux — what we were seeing was Voldemort turning Nagini into a Horcrux, using the murder of the Muggle as the impetus. Finally, in DH, we learn that Harry is also a Horcrux, which is why he was able to see through the eyes of both Voldemort and Nagini.
Scabbers is another prime example, making the rather unorthodox transition from Sidekick's Pet to Henchman in the third book.
Griphook, who has a total of two appearances throughout the series, six books apart - the first as a classic Underground Bank Cart Pilot, the second as a Temporary Ally In Bank Robbery.
The Black sisters:
Bellatrix Lestrange is on trial with Crouch Jr. in the Pensieve Flashback in Goblet of Fire. By the end of the series she's the most prominent and loyal Death Eater.
Draco's mother has been mentioned since Philosopher's Stone and makes her physical appearance in Goblet of Fire. However, it isn't until Half-Blood Prince that we learn her name: Narcissa, and the fact that she is Bellatrix's younger sister. She is responsible for making Snape do the Unbreakable Vow that ends with him killing Dumbledore and in Deathly Hallowstakes a decision that proves critical to Voldemort's defeat.
Andromeda Tonks is mentioned in Order of Phoenix as Sirius' favourite cousin and Tonks' mother. She makes her debut in Deathly Hallows and at the end of the series, becomes her grandson's primary caretaker, following Tonks and Lupin's deaths.
Grindelwald, who was mentioned briefly in the first book in Dumbledore's Chocolate Frog Card description as an evil wizard whom Dumbledore fought. The last book gives him a lot of backstory, including the revelation that he was once Dumbledore's friend.
Madam Rosmerta is a Bit Character in Prisoner of Azkaban and Goblet of Fire. She suddenly gains prominence in Half-Blood Prince as it's revealed that Draco has been controlling her with an Imperius Curse for the whole year as part of his plot to kill Dumbledore. The opal necklace and poisoned mead? All given by Rosmerta.
Both Broderick Bode and Sturgis Podmore were offhandedly mentioned throughout OotP, and were thought to be the most insignificant of characters until Hermione brilliantly pieced it all together and figured out what had happened. (Bode even had a blink-and-you'll-miss-it introduction in the previous book.)
Rufus Scrimgeour is offhandedly mentioned by Order of the Phoenix members in the fifth book before taking over as Minister for Magic in the sixth.
The wandmaker Gregorovitch was mentioned by Ollivander in the fourth book, and serves a brief but important role in the seventh as one of the former owners of the Elder Wand.
Though fan attentiveness paid off in Half-Blood Prince when Harry gets the fake locket with a note by R.A.B. Fans were attentive enough to guess that R.A.B. was Regulus Black, the brother of Sirius briefly mentioned in Order of the Phoenix - mentioned all of twice in the series up to that point; we're talking some very attentive fans here - based on the fact that Regulus was mentioned to have run off to be a Death Eater, then was seemingly killed by Voldemort for getting cold feet, which would have made him a good fit though fans weren't told Regulus's middle name. In fact, fans sussed this with so little trouble, some fans argued against it because it was too obvious. Rowling's response to the theory was "that's a really good guess."
Related to some of the others, make sure you read the Black Family tree in the beginning of the fifth book very carefully, there are a lot of very important names on there.
In a party scene in Arrow's Flight, the second of the Heralds of Valdemar novels, a throwaway line mentions Queen Selenay sitting next to a Herald with streaks of white in his hair at either temple. This turns out to be Herald Eldan, who has a significant role in By the Sword.
Similarly, both Herald Lavan Firestarter (Brightly Burning) and Herald Vanyel (The Last Herald-Mage) are mentioned as historical personages long before Mercedes Lackey wrote a book/trilogy about them.
Sarah Caudwell's Hilary Tamar mysteries have a tendency to mention elderly relatives of the protagonists in passing who will become crucial to later books in the series. The first book, Thus Was Adonis Murdered, mentions that Cantrip has an uncle who's a Second World War veteran; he ends up providing some crucial exposition and a Big Damn Heroes moment in the third book The Sirens Sang of Murder. The Sirens Sang of Murder in turn mentions Julia's Aunt Regina, who narrates a sizable fraction of the fourth book, The Sibyl in Her Grave.
Most notable is Thomas Theisman, who shows up in The Honor of the Queen and has a minor role (well, not that small, but still, he's not even a secondary character). Then he appears briefly in The Short Victorious War and Flag in Exile, presumably due to the Law of Conservation of Detail, and takes a gradually bigger role in the narrative, until he finally overthrows the evil government of his own country, restores the truly democratic one that hadn't existed for two centuries, beats the crap out of the Royal Manticoran Navy, rides to their rescue with his navy after they've had the equivalent of Pearl Harbor pulled on them and then signs a military alliance with them, shares a flag deck with Honor Harrington herself, then does some more awesome stuff... and he probably hasn't finished, as he has ascended to be a series anchor in a World of Badass.
Honor's mum, Allison Benton-Ramirez y Chou Harrington. Mentioned briefly at the beginning of the same book as Theisman, she becomes more and more relevant from In Enemy Hands on. Not to mention that by A Rising Thunder we find out that her brother is a Very Big Fish in the Beowulfan government. Of course, this branch of the family was mentioned earlier, but still...
In the first book, Katniss has a flashback when she meets with a female Avox serving her in the Capitol and Peeta excuses her by noting that it was because she looks like Delly Cartwright, their friend back in District 12. The Avox is later revealed in Mockingjay to be named Lavinia and is killed by the Capitol to torture Peeta during their attempt at hijacking him. Delly Cartwright, meanwhile, also debuts in the same book to help Peeta fight off said hijacking.
Johanna Mason is mentioned when Katniss is recounting the past tributes in the first book before making her first appearance in Catching Fire.
Also from the first book, after Katniss shot an arrow at the Gamemakers she said that one of them tripped backwards into a punch bowl. She has a little interaction with him during her pre-Games interview, then he is never mentioned again. Until the next book, Catching Fire, when she formally meets him and finds out that his name is Plutarch Heavensbee and he is now the Head Gamemaker. At the end of Catching Fire, she finds out that he is also part of a Capitol rebel group, and helped orchestrate the plan to break the victor-tributes out of the Quarter Quell arena. He then becomes a major character in Mockingjay and a significant player in the war effort for the rebels.
In Death series: At one point in Loyalty in Death, an old man mentions how he has met with two guys with dead eyes who are just the muscle and working for terrorist organization Cassandra. The two guys become the murderers to hunt down in the Survivor in Death story. In Judgment In Death, a blonde-haired assassin working for Max Ricker kills off a mook who was going to reveal a lot of details. The blonde-haired assassin turns out to be Max Ricker's daughter and she becomes the murderer to hunt down in the Promises in Death story.
The third chapter of In Desert And Wilderness randomly introduces two nice Englishmen. Who go on to rescue the protagonists from their final predicament.
John Dies at the End: Amy turns out to be this. Early on in the novel we find out she went to high school with Dave and is Big Jim's sister, but then she disappears until Book 2. She returns after Dave and John find out Korrok has been kidnapping her in her sleep. Everything comes back around when Dave begs for her to be saved, which is likely the reason Monster Dave exists in the first place...
In The Long Earth, Rod Green is first introduced as a son of a family which intends to colonize a parallel Earth. Unfortunately, he cannot "step" to parallel Earths, so his family abandons him. He shows up a couple more times where it becomes apparent that he resents his family for his abandonment, but only features prominently when at the end of the novel, where he blows Madison, Wisconsin up with a nuclear bomb.
Not to mention Boromir's passing references to his father, the Steward of Gondor, who becomes a major character in The Return of the King.
Gandalf mentions Aragorn and the Nazgûl in the second chapter. Gandalf even points out the fact that he mentioned the Nazgûl to Frodo before any of the characters even met them.
The Necromancer is given a passing mention in The Hobbit. It is only in the sequel that he becomes important since he is actually the reborn Sauron.
Iny Lorentz likes this trope quite a lot.
In the second book of Die Wanderhure series, there is a grumpy unpleasant woman abandoned by her lover who later disappears from the plot and is practically forgotten. In the middle of book three, she is revealed to be several thousand miles away from Germany, a Tatar khan's favorite wife who pretty much saves the day for the heroine.
In Die Pilgerin, Aymer de Saltilieu makes his first appearance as a minor landlord whose whole task in the plot seems to be a) prove to Tilla that sometimes Evil Is Sexy b) save the pilgrims from his bastard of a cousin. The next time he appears in another region of France, mixed up with the local politics, and then he pops up in Spain as a commander in a decisive battle where Sebastian and Starrheim earn their glory as Big Damn Heroes.
Lunar Chronicles: It's mentioned that Princess Selene of the Moon died in a fire as a child, with only a few burned bits of her body left. It turns out that cyborg protagonist Cinder is in fact Selene, as she was smuggled to Earth and rebuilt after her evil aunt tried to murder her.
Nadir Khan may be an inversion. He has a fairly important role as Amina's first husband before disappearing from the story. He shows up a fair bit later as Qasim the Red, whom Amina is seeing secretly. However, his only contribution is as a living plot device, by motivating Saleem to get them to break up.
Saleem's uncle Mustapha is so irrelevant for most of the story that Saleem barely even describes him or his children. However, towards the end of the book, he briefly gives Saleem shelter after the rest of the family is killed. It's also implied that he helped the Widow track down the midnight children.
When she's initially introduced, Parvati-the-witch is merely Saleem's most vocal supporter during the Midnight's Children Conference, but Saleem makes no attempt to hide her later importance. Towards the end of the story, she encounters him in the flesh at Bangladesh, and saves his life by smuggling him back into India.
Mary Pereira. She runs away after her confession to the Sinais, and Saleem says he doesn't know what happened to her after that, implying she's gone from the story for good. At the very end she's revealed to be the manager of the pickle factory where Saleem works in the present day, who took him under her wing after all his other friends and family were gone.
Ezra Jennings from Wilkie Collins' The Moonstone is the assistant to Dr. Candy, himself a side character. Both of them, but especially Jennings, are more important to the plot than the reader might first think.
In Native Son, Bigger sees Buckley's face on a campaign poster early in the story, long before being prosecuted by him for rape and murder.
In Tad Williams's Otherland series, the side plot involving Olga Pirovsky is treated with a great deal of significance even though it's not initially apparent how her mysterious headaches have anything to do with the main story. Even when she's tasked by Sellars to infiltrate the headquarters of J Corp, it seems like her role is fairly straightforward. Then comes The Reveal.
A few of the main characters in The Pale King go unnamed in their introductory chapters.
In Red Storm Rising, the Soviet Union, suddenly faced with a crippling oil shortage, decides to conquer the Middle East for oil. To do this, they first need to eliminate the threat NATO posed to the operation. Their plan was to detonate a bomb within the Kremlin, killing several staff members and 8 children from the city of Pskov, then blame it on West Germany and invade, hoping that the other western nations would object to being bled white to defend what they would see as a terrorist regime. The funeral is described in great detail, and the viewpoint character of the segment, a non-voting politburo member named Sergetov, focuses on a grief stricken captain of paratroopers, whose daughter's body was so mutilated that her face was draped in black silk for the open-casket ceremony. Near the end of the book, the chairman of the KGB and Sergetov join up with the most senior surviving Soviet general (most of the rest had been shot for failure) in a coup to prevent the deployment of nuclear arms at the battlefront. After taking power, the general turns to the KGB man and the following conversation takes place (paraphrased):
General: By the way, Comrade Lidov, have you met my new aide? He had a daughter in the Young Oktoberists. KGB Chief: Your point? General: His regiment is based in Pskov. Aide: For my little Svetlana, who died without a face. (fires)
Matthew Reilly tends to introduce these early in the book when they become useful.
Ice Station has Trever Barnaby, Jack Wash and Chuck Koslowski mentioned in Schofield's thoughts about his mission. O. Niemeyer also turns up early in an investigation about the events of the book and is currently MIA, but turns out to be a subversion because he died in a plane crash.
Temple has Will's brother Martin Race, supposed to be working with the team remotely.
Six Sacred Stones introduces Jack West's brother-in-law and previous neighbour in a flashback.
In Elizabeth Honey's Remote Man, the book's antagonist is introduced in Chapter 5 as an unpleasant American tourist who buys seven paintings from Ray's gallery in Arnhem Land. A few chapters later Ray's daughter Kate comes to suspect that he was responsible for stealing a rare python, based on the fact that her cousin Ned unwittingly told him where to find it. About halfway through the book, Ned learns that he's a retired Hollywood stunt driver running an international smuggling operation.
The Reynard Cycle: In Defender of the Crown, Reynard follows the trail of an assassin to the garden of a herbalist, where he meets with an assistant. It is later revealed that the herbalist is Hermeline, Reynard's scorned ex-lover, who uses the opportunity to sell Reynard poison rather than a sleeping drug. As a result, Reynard ends up accidentally killing the Queen.
Romance of the Three Kingdoms inconspicuously introduces Sima Yi as Cao Cao's secretary, and you might not expect much from him in a novel with Loads and Loads of Characters (especially considering how the founders of the eponymous Three Kingdoms are introduced). However, Sima Yi eventually becomes Zhuge Liang's rival, before staging a coup during the reign of Cao's great-grandson. His grandson becomes emperor.
Emile Zola loved to use this trope in his Les Rougon-Macquart series. A character mentioned in passing in book one and described by his father as a forgettable good-for-nothing shows up as the main character in books 10 and 11. Another one mentioned in passing in book 3 is the main character of book 14. The pattern repeats itself throughout the books. Things get even more confusing when you find out that the books do not follow in chronological order and that the timelines of most of them intersect in one way or the other. Trying to keep up with who is doing what and is important in which book can become a nightmare.
Safehold: The earlier books have occasional mentions of a mythical folk hero, Seijin Kohdy. And then comes book 8, Hell's Foundations Quiver, where it turns out he was a real person and there's an ancient secret society who venerate him...
Early in The Secrets of Drearcliff Grange School it's mentioned as a passing oddity that a student named Enid ffolliott has been missing for two years but is still called on during the weekly roll-call because she never officially left the school. She makes a dramatic reappearance later in the story.
In J.C. Hutchins'a 7th Son trilogy, characters like Special K, Peppermint Patty and Klaus Bregner get throwaway mentions only to become very important later on.
Also done in books two and three of Cinda Williams Chima's High Fantasy smash hit, The Seven Realms Series. In book two, Han convinces Action Girl Catarina to study at the temple school at Oden's Ford, an area of the school known for producing the most refined maidservants in all of the Seven Realms. In book three, Han needs someone he can trust near Raisa, but someone that can also defend her. Turns out those skills Cat got came in handy.
Katia, alluded to as being in Bonn throughout the first half of the story, as well as the object of Witkacy's musings on their Ambiguous Situation together, returns halfway through and comes to Witkacy for help.
The hospital ghost who clues Witkacy in as to the nature of the wraith comes back to tell him about the spiral and what it does to ghosts.
In Silas Marner, the character Godfrey Cass is seemingly of no direct importance to the main plotline, until his brother robs Silas, and then his secret daughter ends up in Silas's care. Small world!
In The Silmarillion, Lúthien Tinúviel is introduced off-hand by Tolkien simply as the daughter of Thingol and Melian, but later, as we all know, plays a major part in retrieving a Silmaril by owning Sauron's face and tricking Morgoth himself and proving herself to be totally badass.
Smaller & Smaller Circles: Councillor Cesar Mariano, who sponsors free dental checkups for street children, is introduced as simply a leader in the local government. Fr. Saenz then notices one of his vans advertising these dental services, and then later figures out that the murder victims, also street children, might have had dental records thanks to Mariano's program.
A Song of Ice and Fire has egregious numbers of Gunmen. An outstanding example would be Lord Beric Dondarrion, who was dispatched in the first book to deal with outlaws, only to show up two books later. Almost every important character, if not introduced, was at least mentioned a book prior.
In GoT, Cersei exiles Ser Barristan from court for political reasons. Several books later, he shows up to save Daenerys' life and pledge his loyalty to her... which leads to him becoming a POV character.
Also in GoT, Stannis Baratheon is mentioned a lot in Ned Stark's chapters, and Ned agonizes about why he's not in King's Landing nor responding to Ned's requests to return (had he been there, things might have gone a lot differently for Ned). In the next book, it's revealed exactly what he's been up to and he becomes a sort of Hero Antagonist for Tyrion and the other Lannisters.
The Faceless Men are described in passing as a legendary guild of assassins, before the protagonist Arya joins them. Her training with them forms a lot of Arya's character arc.
Houses Bolton, Greyjoy, Tyrell, and Martell are all mentioned or given minor introductions early, but gain major plot importance later.
Wex Pyke appears in "A Clash of King's" in Theon's chapters, as a mute bastard who is made Theon's squire and accompanies him to Winterfell. In "A Dance with Dragons" it turns out he survived the Sack of Winterfell and has revealed to Lord Manderly the truth of what happened and knows where Rickon Stark is hiding.
Star Wars Legends: A head of one of the smuggling organizations with a subplot in The Thrawn Trilogy has bodyguard, and in one line he calls her Shada. She's good at her job. Zahn later wrote a short story called "Hammertong", in which she was an all-around agent who was one of the many people who helped get the Death Star plans to Leia. In the Hand of Thrawn duology written five years after The Thrawn Trilogy, Shada Du'kal accompanies Talon Karrde on his search for a certain document, and has her own Mystral subplot.
Double-subverted with the crazy old man Lift chats with after her first visit in the orphanage. Later, it turns out that Nale's acolytes believe him to be the Lightweaver they're hunting. Turns out he's not; then it turns out he's actually a Dysian Aimian who kills the acolytes with contemptuous ease and gives Lift some crucial information.
Stump, the old woman who's mean to Lift in the orphanage, later turns out to be the person Nale was looking for.
The guard captain Lift surprises at the start of the story turns out to be important when Nale is visiting witnesses, looking for Lift.
In The Sun Sword, Ser Laonis di'Caveras saves Lissa's life in the opening chapters of the book. He turns up for an unnamed cameo appearance at the end of the second book, and then four books later he saves Diora's life. The same woman who, as a very young child, accused him of being less than a man for initially refusing to save Lissa's life.
The fact that Gingema had owls is mentioned briefly in book one. In book two, the oldest and wisest of them becomes Non-Human Sidekick for the new villain and a recurring character for books four through six.
In book one, Scarecrow tells Ellie that a passing crow taunted him for having no brains, and that's how he came to the idea that brains are the best thing in the world. By book two, the crow too realizes that it was her who inspired the new monarch of the Emerald City, reacquaints herself with him and becomes one of his most trusted advisors. She acts as messenger for the heroes, becomes Minister of Communications (and once serves as regent when Scarecrow is imprisoned), and generally takes the role of The Cynic in the heroic team.
Ramina the Mouse Queen acts like a minor Deus ex Machina, not different from the other good witches, in the first book. But she's the only good witch to make prominent appearances throughout the series, and in books five and six her mice play part in some crucial plot turns.
The Leapers, nothing but another problem for the heroes to deal with in the first book, turn into a major Chekhov's Army three books later as Urfin Jus becomes their leader.
A Winkie mechanic plays a small (though not unimportant) part in the last battle of the second book. Later he becomes a major recurring character and the "Science" side in any Magic Versus Science conflict.
A less noticeable example, Gurrikap appears in the prologue to book three as the founder of the Magic Land. He appears again in the prologue to book five, and this time it's more important to the backstory. Also his long-abandoned palace becomes the titular plot location in the sixth novel.
Temeraire: Laurence is impressed enough by the valor of one enemy French soldier that he compliments his fighting and takes care to capture him alive. Later, he's revealed to be the nephew of a French ambassador, who becomes a Friendly Enemy to Laurence and helps him out on multiple occasions.
Prince Oswin notes in the very beginning of Terra Mirum Chronicles that anyone useless is disposed of during a revolution. So why is Erebus still alive after the Nightmare Queen has everything she should need from him?
Those That Wake has Isabel, who appears early on and shows up again in the middle of the book.
In The Three Hostages, when the protagonist Richard Hannay is still having the wool pulled over his eyes by the Villain with Good Publicity, Hannay's friend Sandy mentions that he's never quite trusted the man because of the way he treated Sandy's friend Lavater. Lavater turns out to have a larger role in the story than is immediately obvious.
Tortall Universe: Beka Cooper mentions King Roger's new young wife in the first book, Terrier. In Mastiff, it's revealed that she is basically responsible for his actually making himself a responsible king instead of a rake with a crown, as he didn't want to embarrass himself when she kept asking him questions about the management of the realm. (Of course, this pisses off all the nobles who were quite happy to have an Idle Rich king, so Beka meets Jessamine after her son's kidnapping in an elaborate usurpation plot.)
The Necromancer is mentioned off-handedly several times in the first three books as someone else's problem. In book four, he becomes heroes' problem.
In the first book, Mogon is mentioned in passing as the sister of Quethnethog leader. In the following books, she becomes the Warden herself and is one of more influential allies of the heroes.
The golden bear that John Crayford rescues and chats with briefly in book two turns up again in book three and reveals himself to be the top leader of the golden bears in the Adnacrags, now with acquired fondness for humans.
Trapped on Draconica: Subverted. Daniar busts a drug runner early on in the narrative and he shows up later working for the Big Bad and out for revenge. Daniar busts him again without any trouble then or later.
Early on in A Tree Grows in Brooklyn, Sissy chats with a nameless policeman who tells her his wife is an invalid. A few years later, her sister Katie meets a policeman named McShane, whose wife has tuberculosis. He keeps track of her, and they marry several years later after both their spouses have died.
Undefeated Bahamut Chronicle: Alma Kilzrake, aka Almatea Atismata. She's first mentioned in the very first volume, as Lisha's younger sister, and believed to be dead. She makes her first appearance fifteen volumes later and becomes a major character.
Vampirocracy: Harold and Elizabeth Anderson, seemingly two inconsequential potential clients of Leon's PI firm, turn out to being the key to finding out who killed several vampires.
In Sandy Mitchell's first Ciaphas Cain short story, we are introduced to Cain's rather smelly and loyal aide Jurgen, who doesn't look like he'll have much relevance except for jokes on how he puts people off with his atrocious hygiene standards. Later on, it turns out Jurgen is an extremely potent "blank," someone who negates psychic powers and harms daemons simply by being in proximity to them, and his becomes a constant and critical plot point throughout the rest of the novels. His absolute loyalty and obedience to Cain also play important roles.
Something of a bizarre usage in Dan Abnett's novels, where his early Gaunt's Ghosts stories namedrop characters who he would later develop in future novels, but his Word of God reveals that he hadn't actually planned so far ahead. For example, comments made regarding Inquisitor Gideon Ravenor, who would eventually get his own novels, from the Gaunt's Ghosts Omnibus The Founding:
"... first mention of Ravenor (who could have guessed where that would lead to)..."
In H. G. Wells's The War of the Worlds, an unnamed artillery gunner appears in the main character's house and informs him of the Martian fighting machines, They go to a town together which is later attacked by the Martians, and he disappears in the chaos. Later he re-appears, speaking about how they can build a whole world underground. Over the course of the chapter, this becomes pivotal to the unnamed main character's decision to kill himself by running up to the Martian fighting machine— only to discover it dead, leading to the final chapter of the book.
When first introduced in Warrior CatsInto The Wild, Littlecloud is just a small, seemingly unimportant apprentice from ShadowClan, but later becomes the ShadowClan Medicine Cat after Runningnose and a good friend to Cinderpelt.
Welcome To Wonderland: P.T.'s friend Jack Alberto serves this role in "Sandapalooza Shake-Up". In the book, he suggests to P.T. and Gloria that they have the Wonderland motel host a metal detector treasure hunt on the beach. Near the end, they recruit him to use his metal detector to find the Twittleham Tiara hidden in the sand sculpture that Travis and Darryl made.
In The Wheel of Time every named character in the book will return with greater importance latter on. Every. Last. One. It's even justified by the fact that the three main characters bend chance, circumstance, and the fabric of the universe itself simply by existing.
In When You Reach Me, the laughing man seems to have no purpose in the book, but in the end, we learn that he's one of the main character's friends who has traveled back in time to save one of her other friend's lives
In The Wild Ones, Gayle appears to be another obstacle in Kit's path to prevent him from getting the Bone of Contention. However, he makes a deal with Gayle off-screen and she is never mentioned again during the book. That is, not until she shows up at the end to eat Sixclaw, and to scare all of Titus' army into running away.
In the first Wings of Fire series, Darkstalker is mentioned offhand as a boogeyman-esque figure whom most dragons believe is long dead. In the second series, he is revealed to be alive and becomes the protagonist's mentor.
Dragonbit Vipers are briefly mentioned at the beginning of The Brightest Night as part of a ploy by Addax to cause panic. At the climax Blister murders Burn with one.
Cirrus the SeaWing was first seen in The Lost Heir as a minor character. But he becomes more important in Escaping Peril, where Cirrus is actually a RainWing named Chameleon in disguise, and he's Peril's father.
The story opens with Safi explaining in narration that's she's preparing to rob a carriage because she lost all her money in a card game to a guy she nicknamed the Chiseled Cheater. He appears in book two and it turns out he's a captain of the Hell Bards, hunting for her.
Ryber, Kullen's girlfriend, pops up a few times in the first book and vanishes for most of the second, only to reappear in the epilogue and reveal that she knows a suspicious number of things about Merik's affliction. The prequel Sightwitch explains what her deal is.
Early on, Iseult mentions that when she ran away from home, she was rescued by a Carawen monk. The same monk appears later, and it turns out she's Merik's aunt and resident Ms. Exposition.
In the first book, Safi briefly passes by a sailor who calls her "Matsi-loving scum" for her friendship with Iseult. In the second book, she meets the same man in prison and he brings her up to speed on Merik's situation, fast-tracking Safi's character development.
Merik's first scene has him at a dinner party, pondering with annoyance the fact that everyone around him is gossipping and partying and completely ignoring important news, such as the rumours of a new king uniting the tribes prowling the territory of the Former Republic of Arithuania. This new king is none other than the Big Bad.
In the Young Wizards books, there are a decent number. Two of note are Ponch, who in an incidence of Chekhov's Gun does indeed get stranger the more time he spends with Kit, as wizards pets do. He crosses into and even creates alternate realities and is implied to maybe be God. Who apparently likes palindromes. The other is Biddy in A Wizard Abroad who is the local smith and farrier and turns out to be one of the Powers That Be and helps reforge a powerful artifact that has been lost.