This happened somewhat to Yue Ayase in the Mahou Sensei Negima! anime. While it was just barely alluded to in the Field Trip arc (which is where they likely drew it from), the love triangle plot come into play much later in the manga.
Valkyria Chronicles anime adaptation has promoted Faldio to love interest in order to introduce a Love Triangle. Not only totally unnecessary considering the existing romantic subplot, but it's become a rather large Romantic Plot Tumor. For instance, a rather epic battle in the game against an enormous overpowered tank was completely avoided in the anime to allow Alicia and Faldio time to flirt.
In the Tokko manga, Ranmaru and Sakura don't show any specific romantic interest in each other, but in the anime they develop feelings for each other and become love interests.
The closest thing to an original canon for Black★Rock Shooter is a music video of the eponymous character fighting Dead Master. Once people started making actual stories, Dead Master ended up as the love interest in one manga, the "other self" of BRS's "other self"'s love interest in both anime, BRS's mother figure in the gag manga, and nonexistent in the game.
In Trigun, Meryl and Milly care deeply about Vash and Wolfwood, but it doesn't go beyond anything platonic. In the anime adaptation they become their respective love interests ,and its even implied that Wolfwood and Milly have sex with each other later in the series.
Bleach: Word of God has stated it's not a romance manga, which is why the main characters have no romance and any romance that does get covered is either in the background or part of another character's back story. However, in the anime filler arc, the Gotei 13 Invasion, the anime team demote all the main cast and even the secondary cast to extras in favour of their filler villains and heroine and their filler evil clones. To ensure there's romance in the arc, they make Kon the hero of the arc (demoting even the main character) by promoting him to the filler heroine's love interest (even changing his back story to accommodate her).
Liz Sherman from Hellboy. In the comicbooks, she's Hellboy's co-worker and friend, and something of a little sister. In the movies, she ends up boinking Hellboy. It did give her some character, since in the comics, she's mostly a Flat Character that the author has no idea to do with and only narrowly avoided killing off. (Her first four comics appearances all involve an antagonist sucking her power out for his own ends, which was apparently the only plot Mignola could think of for her.) Mignola actually okayed the Re Tool of her character.
Interestingly, in the comics Liz actually has something of a romantic sub-plot with Abe Sapien, professional fish-person and Hellboy's best friend. Still, this sub-plot is extremely subdued, but it does get played up a bit in the animated adaptations.
In Ultimate Spider-Man, Kitty Pryde is one of Spidey's girlfriends briefly, giving him a relationship with someone he doesn't have to constantly worry about the safety of. They do break up eventually.
Somewhat (and very surprisingly) averted in the movie adaptation to I, Robot. Both the male lead and the female lead are very attractive, yet they never get romantically involved. Well, not explicitly, but there are hints at it, and Susan Calvin in the short stories was an elderly, celibate misanthrope after the first few Time Skips. She gets upgraded to a Hot Scientist, and at least a potential love interest, but thankfully they focused on the robots and the related ideals.
For the movie adaptation of Watchmen it was originally planned to give Rorschach a love interest since his actions drive the whole story. But the idea was dropped.
The 1960 film adaptation of H. G. Wells's The Time Machine is an Alternate Trope Namer. In the book, an Eloi woman named Weena shares a close relationship with the Time Traveler, but not a romance, at least not overtly. Largely because Weena, like all Eloi, was a child-sized androgynous-looking creature mentally on the level of an eight-year old. However, the film turns Weena into a love interest, looking human except still with the mentality of a child.
The 2002 film goes further: not only was Weena replaced with a love interest named Mara and the Eloi made even less childlike, but the Time Traveler was given an entire backstory of building the machine as a way to save his girlfriend from being killed by a mugger.
Isabella of France in Braveheart was in France and ten years old at the time of Wallace's rebellion. Particularly unfortunate as her romantic subplot is at cross-purposes with some of the most powerful moments in the film.
The 1966 film version of Ray Bradbury's Fahrenheit 451 not only had Clarisse survive, but also had her become a sort of love interest for Guy.
Not sure what Bradbury thought of the "love interest" part of it, but he considered rewriting the book to have her survive, and effectively did so with his theater adaptation.
Ellie Sattler is given a larger part and promoted to Alan Grant's love interest in the film version of Jurassic Park.
In a rare subversion, though, her role as a love interest is only subtly hinted at and the two don't end up together. By the third movie, Ellie Sattler is married (presumably happily) to a different man, and is raising a family. Even her larger role in the first film may be more about giving the female characters something to do other than be The Chick (Lex is likewise made important and capable in the film, largely by switching her age and most of her other character attributes with those of her brother from the book.)
In a move that savvy movie goers could have seen coming, the new Star Trek movie does this to Lt. Uhura. What they probably didn't see coming was that she's Spock's love interest, not Kirk's. Which is less surprising if you keep in mind a couple of early episodes of the original series in which she was blatantly flirting with him (and, more strangely given his later characterization, him with her!). Of course, given the time period of the original series, the only reason Uhura wasn't officially anyone's love interest was probably because of the interracial relationship thing. Word of God did once say he had dropped the idea of Spock/Uhura in the original series because he feared it would reduce Uhura to being viewed as nothing more than "Spock's other half" instead of letting her being a character in her own right.
Some of the negative reaction the new pairing recieved is because it gets in the way of Spock and Kirk, a pairing so famous and long-cherished in the fandom that its usual notation gave us the term "slash".
Conversely, the movie didn't try to pair Spock up with Nurse Chapel, who had a crush on him in the series that was never developed due to the network objections (they didn't want the main male characters in any long-term relationships, which ruled out both Spock/Uhura and Spock/Chapel, but which does explain why there were episodes that displayed Spock apparently flirting with both women (at different times) before it was knocked on the head).
According to the script-writers, they were going with that pairing in an early draft. But when they needed someone besides his mother for Spock to interact with in a caring way, they thought people would take Uhura more seriously compared to Christine (which a lot of fans have agreed on, considering Christine's Stalker with a Crush behaviour in the original series).
The Film of the Book of The Chronicles of Narnia: Prince Caspian creates a romance between Susan and Caspian. Guess they didn't want to wait one more movie for Caspian's marriage to Ramandu's daughter. Fortunately, it's only a mild romance, limited to flirting and a goodbye kiss.
Trillian in the film version of The Hitchhiker's Guide to the Galaxy. In the original, Arthur met her at a party and tried to (clumsily) chat her up, only for Zaphod to sweep her off her feet and take her into space; when they meet up later on, their relationship is purely platonic. The movie changes this to Arthur missing his chance at True Love by being too wimpy, inspiring him to be more assertive when Trillian gets kidnapped (also helped by introducing a new love interest for Zaphod).
Starship Troopers does this with "Dizzy" Flores who, in the book, is a guy in the same platoon as Rico, and is only mentioned in the first chapter, due to the fact that he dies during a drop, and is not romantic at all. (Per the other wiki and, you know, the book.)  Of course, Dizzy gets an upgrade with boobs and boinked. It's good both ways.
In the movie of the children's book Tuck Everlasting, the main character, Winnie, is a teenager and thus old enough to have a romance with a Flying Dutchman who's been seventeen for ages. In the book, she was a preteen with perhaps a slight implied crush on him and nothing more.
Inverted in Angels and Demons: Vittoria is in bed with Langdon by the end of the book, but in the movie, romance is never hinted at.
This is actually inverted in the movie adaptation of Psycho. In the original novel Lila and Sam become romantically involved after Mary is killed and they try to solve her murder. Hitchcock made their relationship platonic in the film, because it would be gross otherwise. In the 1982 sequel, though, Lila has married Sam and had a daughter with him, called Mary.
One of the changes made in the film version of Kick-Ass. In the film, he ends up boinking with the girl he's got a crush on. In the novel, he ends up crumpled on the floor at high school due to a Groin Attack from her boyfriend.
Lord of the Rings. Okay, so Arwen was present in the books as a love interest but she wasn't present a whole lot. The movies upped that so she made an appearance (even if it was just in a dream/flashback/whatever) in every single movie.
In a sense it was the reverse of this trope- in the book she's a love interest who barely exists before she arrives for her wedding and has no obvious plot relevance. (She was roped in for this purpose at the last minute, apparently.) The films apparently tried to gel her into the story proper.
There was actually justification for Aragorn and Arwen getting together, but... it was in one of the appendices. Definitely not the only relevant detail to be shoved in there — really, those appendices are big enough to fill an entire book by themselves, and that's before any attempt at turning all those notes into a proper story.
The 1960 film version of Edgar Allan Poe's Fall of the House of Usher makes Madeline the fiancée of the narrator.
The film version of Queen of the Damned made the main subplot a romance between the two main characters Lestat and Jesse, who, in the book, do not speak. One of the many things altered from the book is the identity of Jesse's maker. In the book, it's her "Aunt" Maharet (a distant ancestor-turned-vampire). In the movie, it's Lestat. This was obviously meant to reinforce the bond between the characters, which was never there in the book.
Inverted in the film Shooter which is based on the Stephen Hunter novel Point of Impact. In the book Bob Lee Swagger becomes romantically involved with the widow of his old war buddy. In the movie the two become friends and allies, but they do not fall in love with each other.
The 2009 film of Land of the Lost has a truly bizarre version of this. Rick Marshall has a romantic relationship with Holly, who was his prepubescent daughter in the original TV show. The movie makes them unrelated and ages her up, obviously, but one wonders why they even bothered to call her "Holly" at that point.
The Man Who Fell to Earth promotes the middle-aged hotel maid Betty Jo, who nurses an unrequited crush on Alien Among Us Thomas Jerome Newton and is one of his only confidantes in the novel, to an out-and-out love interest. Renamed Mary-Lou, she's also de-aged to her mid-twenties when their relationship, which plays out over several decades, begins.
Inverted with the Disney version of Peter Pan, which removes virtually any trace of romance between Peter and Wendy present in the original book. Those only familiar with that version mistakenly believe this trope to be at play with the 2003 film version which was adapted closer to the novel.
The Giver: Although Jonas had a crush on Fiona in the original novel, she didn't do a whole lot. They eventually drift apart and he ends up with someone else in the sequels. In the film, she becomes an example when she stops taking the emotion-suppressing injections at his behest and admits she's in love with him too. They share kisses and she aids in his escape.
As reported in Orson Scott Card's commentary on the Ender’s Game audiobook, the main reason that the Ender's Game movie was in Development Hell for so long is that so many producers wanted to age Ender to a teenager and/or give him a love interest, and the author rightfully refused to sign any contract that allowed anything of the sort.
Leaving aside the unfilmableindescribable nature of a lot of his stuff, this is probably the recurring problem with attempts at filming the works of H.P. Lovecraft, who was a very unromantic guy and didn't put any love subplots into his stories at all. Apparently, Guillermo del Toro's upcoming version of At the Mountains of Madness will avert this trope, which is why it took him so long to get the permission of producers.
Adaptations and pastiches featuring Sherlock Holmes almost always do this to Irene Adler, the one person to outsmart and upstage Holmes, who Holmes refers to as "the Woman". Canonically, Holmes only ever had one brief meeting with Adler, and his feelings for her are more along the lines of grudging admiration. There's also the fact that the one story that Adler appears in revolves around her running off to marry someone else. Even if Holmes was into relationships, Irene seems pretty stoked with her hot lawyer husband.
Most adaptations of Dracula make Mina Murray Harker (or her equivalent, as sometimes her role and that of Lucy Westenra are flipped or combined) into the title character's love interest, a trend that's often attributed to Bram Stoker's Dracula (the film), which made Mina the reincarnation of Vlad Tepes' bride Elisabeta. However, a similar incident occurred in the 1970 TV version, where Lucy looked like Dracula's lost love, and even the 1950s Hammer Horror version referred to Dracula in the posters as "the terrifying lover who died- yet lived!" In the book, there was indeed a vague indication that Dracula intended for her to become his queen, but Mina regarded this as A Fate Worse Than Death, complete with all the rape associations that went along with this.
Which was doubly inverted in the movie, where Mina's husband is dead and she has no affair with Quatermaine. Sawyer flirts with her a bit and she did have an affair with Dorian Grey in the past, but it's implied that her years of living as a vampire have left her unable to really love anyone.
In the Novelization of Douglas Adams' Development HellDoctor Who episode Shada, the Fourth Doctor gets some fun sexual tension with a sentient spaceship. In the original her role is simply to be confused by him with a Logic Bomb, but in the book the experience (along with him also teaching her how to time travel in a way suggestively related to him teaching her how to orgasm) makes her curious about the world and eventually fall for the Doctor, who for his part is respectful but not very reciprocal of her feelings - though it's worth pointing out that when he attempts to guilt Skagra about trying to destroy the Ship, his retort (that 'a machine consciousness is worthless') is the This Means War! moment.
Chris and Clare also become love interests for each other, though this was an element that Douglas Adams had intended to put in the story, managed to get elements of in the first couple of episodes, but then got cut thanks to a combination of deadlines and Executive Meddling. So in this case, it's more of a What Could Have Been.
Jack Robinson in Miss Fisher's Murder Mysteries. In the novels he is a Happily Married middle-aged man Phryne has no romantic interest in. Phryne's canon love interest in the novels, Lin Chung, only appears in one episode of the series ("Ruddy Gore").
The play The Solid Gold Cadillac had a relatively low-key romance between Mrs. Laura Partridge and Ed McKeever, even though the narration referred to the couple as "Cinderella" and "Prince Charming," who (of course) were married in the end. The movie version is much more of a romantic comedy, playing up the romance between McKeever and Mrs.Miss Partridge to an extent that the Tabloid Melodrama about the characters is fairly justified.
In the stage musical of Beauty and the Beast, Cogsworth and the wardrobe are portrayed as a couple, but are much more reserved about it than Lumiere and the duster.
In the musical of The Producers the previously minor part of Swedish secretary Ulla is not only expanded into leading lady but she becomes Bloom's love interest and briefly is the center of a one-sided Love Triangle between him and Max.
The musical adaptation of The Scarlet Pimpernel adds a romantic history for the villain Chauvelin and the heroine Marguerite, making the former something of The Vamp, since the latter used to be on his side. In the original novel, they were nothing more than acquaintances in the past and Chauvelin sees Marguerite as nothing but "a tool" now, his faith in her intellect to help him nab the Scarlet Pimpernel and his constant Terms of Endangerment aside.
In Seussical, Horton gets a lover interest in Gertrude McFuzz, a character from another of Seuss' books.
The Wicked Witch of the West and the Scarecrow are promoted to being love interests in the musical Wicked, based on a book of the same name. It seems really Squicky unless you've actually seen the musical ( Wicked Witch Elphaba turned Fiyero, her boyfriend, into the scarecrow to keep him from being tortured to death). After that, it's all just an interesting Alternative Character Interpretation.
In Agatha Christie's own theatrical adaptation of And Then There Were Nonetwo of the characters survive and fall in love.
Lumina and Rick from Harvest Moon. Lumina, the girl who appeared as a Lonely Rich Kid, who thought of you as her big brother and eventually fell in love with you, in "A Wonderful Life", was changed into a love interest for "Special Edition". She's changed from fourteen to sixteen(JP)/eighteen, and looks the same, though. Rick was just ran a tool shop in 64, but was changed to a love interest in Back to Nature For Girl.
When the Takarazuka stage adaptation Phoenix Wright: Ace Attorney was announced, fans who were familiar with the Takarazuka's penchant for adding in romantic subplots speculated wildly on who'd end up as Phoenix's love interest, with guesses ranging from the plausible (Mia, Maya) to the highly unlikely (Edgeworth). In the end, the one who got the promotion was, of all people, Lana Skye (well, they called her Leona, but if it walks like a Lana and quacks like a Lana...), a character who appears in only one case (albeit a fairly long one) and doesn't have any particular history or chemistry with Phoenix.
In the Mass Effect series, Garrus and Tali weren't originally intended as romance options. Fans liked them so much that they were added to the roster of possible love interests in the second game. Funnily enough, if Shepard pursues neither of them, they hook up with each other near the end of the third game.
Crea's route in Duel Savior Destiny mostly replaces the harem route from the original game, though in order to accommodate this change they made surprisingly few tweaks to the original story, which makes Crea not really fit in very well with what's going on.
BIONICLE usually follows the No Hugging, No Kissing rule to the extreme, but in the second and third movies, Matau noticeably flirts with Nokama quite a bit and there are some pretty noticeable hints that Nokama herself is into Vakama. The third movie also involves an Unholy Matrimony plot with the villains. (Other Bionicle media does include this stuff, but pretty much only to keep consistent with the movies. In the case of the Unholy Matrimony, Word of God writes off "marriage" as strictly political in this 'verse.)
Also in the first movie, there are some incredibly obvious hints that Hahli and Jaller like each other, though this was back before the hugging/kissing rule was really brought in.
The fourth movie took place in a world where there's no such thing as No Hugging, No Kissing, and one of the female characters (well, the only one to ever get a spotlight, anyway), Kiina, has shown feelings toward Berix — which Word of God promptly declared to be a show of mere sympathy. However in the sequel novel, Kiina was then promoted to a romantic interest, for real, to the movie's main hero Mata Nui. But as the storyline was still targeted at a younger male audience (who, according to the creators, are still afraid of cooties), and since Mata Nui was an outcast from the no-romance world and also had to turn back into an Humongous Mecha, this "relationship" didn't last beyond one small reference in the novel.
Happened to Spider-Woman in the Iron Man cartoon. Considering the Merchandise-Driven nature of the show, this was assuredly so that they could give Tony Stark a love interest who could get a toy.
The Spectacular Spider-Maninverts this in the case of Mary Jane Watson. In the comics, she's one of several Love Interests for Peter, albeit the one he finally married (until One More Day, at least). In most adaptations, most notably the movies, she's given a greater role while the others are reduced, cut or combined with her. In Spectacular she goes with Peter to a dance in her first appearance, but is otherwise just a friend—the main love interests are Gwen Stacy and Liz Allan (plus some heavy-duty flirting from Black Cat when she appears). However, there were five seasons planned and according to Word of God, Mary Jane would have eventually been promoted. Notable in that Gwen Stacy would still be alive.
Almost none of the couples in the Young Justice animated series are present in the comic; Miss Martian and Superboy are paired up together despite the former being single and the latter being in a relationship with Wonder Girl II in the comic; the Artemis from the comic is a villain and never had a relationship with Kid Flash; on the other hand, Speedy and Cheshire did get together and have a daughter in the comic. Robin (Dick Grayson) and Zatanna are an interesting variation. In the series, Zatanna is a teenager roughly the same age as the rest of the main cast, rather than being around the same age as Batman. Zatanna has been a romantic interest for Batman in certain continuities. Dick and Zatanna only dated for a short time on the series anyways. The tie-in comic also reveals that he dated Rocket at some point and currently has an ill-defined thing with Batgirl.
Similarly, in Teen Titans, Kid Flash is paired with Jinx, who is also a supervillain with whom he had no relationship in the comics. So apparently he has a type. And it's inverted with Beast Boy and Raven. While in the comics that were out at the time had them in a romantic relationship, in the cartoon they have more of a brother-sister relationship instead. Canonically. Don't tell the shippers.