This character is a hopeless romantic. He's madly, passionately, fatally devoted to the girl he loves. It's just that the girl he loves seems to change from week to week. When one girl rejects him (which she usually does), he is heartbroken at first, but he soon finds a new love interest, and forgets all about the old one.
Unlike The Casanova, who strings many women along at the same time without caring about any of them, the Serial Romeo genuinely believes that each successive love is "The One", and that without her, life is not worth living. Expect this character to be completely blind to the implications of having a new "True Love" every week. If called out on his fickleness, expect him to insist that his current love interest is the only person he has ever really loved, and to dismiss her predecessors as mere youthful infatuations. Expect much eye-rolling from his friends.
Note: Although this trope usually applies to men, feel free to put female examples here as well.
- Claire Stanfield from Baccano! has a habit of falling in Love at First Sight with basically every girl ever, proposing to them in all seriousness at their first meeting. Chane was the first to say yes.
- Nana "Hachi" Komatsu from Nana was hopelessly in love with every boy she met on a serial basis, especially at the beginning of the manga.
- The Pokémon anime has Brock. But it's not (usually) because he gets rejected, but he keeps being stopped and pulled away by Misty (pulling by the ear), Max (after getting it from the former), May (on one occasion), his Bonsly (only a few times with Double-Edge), or his Croagunk (with a Poison Jab, no less).
- In later arcs, Ash's Oshawott and Clemont's Chespin take the role.
- Sailor Moon has all the Sailors relatively boy-crazy but Makoto is easily the one who has definitely had the most relationships. A Running Gag when she was first introduced was to say that a random guy reminds her of one of her old boyfriends.
- In the manga, Minako tops her by a few orders of magnitude. During Codename: Sailor V Artemis kept count of Minako's 'first loves' (as she called them), and the final chapter had him sarcastically congratulate her for her 5,000th first love.
- Martina of Slayers Next falls madly in love with just about every male member of the cast she meets. The fixation on Zangulus at the end of the season lasts long enough to get her married off.
- Sameer from the Bollywood movie Dil Chahta Hai. Unlike his friend Akash, who is The Casanova, Sameer believes that he is truly in love which each new pretty girl he meets, and is heartbroken when they reject him. His despairing parents eventually try to set him up in an arranged marriage with a girl named Pooja. Fortunately, Sameer ends up falling in love with her, too!
- An Abundance of Katherines: Maybe not every week, but Colin has dated more girls named Katherine than he's had years in his life.
- Claire Stanfield from Baccano! has a history of this, literally falling in love and proposing marriage to complete strangers, almost at random. He has a hard time convincing his brothers that his relationship with Chane is any different other than that she didn't immediately say no.
- The Dresden Files novel Blood Rites features Arturo, who has been married (and divorced) thrice and currently has a new beau. People who know him claims that he is capable of intense love towards whoever he's got his eye on at the moment, but has a problem when the magic dies — it is notable that in a universe where The Power of Love is real, Arturo is capable of showing it towards anyone he's interested in. It is revealed at the end that for the first time, he married a plain woman. Perhaps this one would last.
- In some of the Dragonlance Novels detailing the early lives of the Heroes of the Lance, Caramon is described this way. His brother makes some snarky remarks about his tendency to be in love with a new girl every week, though eventually he settles down with his true love Tika.
- The Encyclopedia Brown series has Kid-anova Tyrone Taylor.
- Gossip Girl has two. Nate Archibald and Serena van der Woodsen.
- Howl from Howl's Moving Castle pretty much defines this trope. He goes out, falls in love with a girl, spends an inordinate amount of time and effort wooing her, and then...drops her once she falls in love with him. Repeat on infinite. The first one he fails to do this to is Sophie, who turned him into a Lady Killer In Love and whom he eventually married.
- Bingo Little from P. G. Wodehouse's Jeeves and Wooster series, although he eventually gets married and settles down. Bertie himself could count as a milder example—he falls for one Girl of the Week after another, but generally ends up expressing relief when things don't work out.
- William's older brother Robert in the Just William stories.
- Sansa from A Song of Ice and Fire has shades of this. She falls quite hard for Prince Joffrey at the first hint that their parents are considering bethrothing them. She fantasizes about marrying Ser Loras both before and after Joffrey's true nature is revealed and she falls out of "love" with him, then transfers her affections wholly to Loras' brother Willas on being offered a secret marriage pact to him, without even meeting him. Seemingly the only male she DOESN'T fall in instant love with the moment a possible romantic relationship is suggested is her eventual husband, Tyrion. Not without a certain amount of justification.
- Adolin from The Stormlight Archive is this in spades. Frequently lampshaded by his brother, and several other characters. In the second book he's fine with the prospect of an Arranged Marriage to a woman from an insignificant house he's never met partially because by this point he's offended literally every other eligible woman, and the idea of letting someone else make the arrangements sounds good. Oddly enough, the Arranged Marriage seems likely to become a Perfectly Arranged Marriage, as he and the girl in question fit together quite well. There's a bit of a Running Gag throughout the second book where Adolin will be in the same scene with a random woman and she'll glare at him. At one point, the woman teaching Kaladin's men how to care for their horses starts throwing stones at Adolin without missing a beat.
- Very much the case with Neal from the ''Protector of the Small quartet by Tamora Pierce, although he eventually gets over it and gets married.
- In Will Grayson, Will Grayson Tiny Cooper is constantly swooning after and being heart-broken over guy after guy (after guy after guy). Apart from his gay-ness and large-ness, this is practically his defining feature.
- Gösta Berling spends most of Gösta Berling's Saga falling desperately in love with one woman after another, winning her love, turning her life upside down, and then moving on to do the same thing to someone else about ten days later, until he's eventually forced to grow up.
- Phoebe from Charmed starts out as one but eventually evolves into a Deconstruction. The endless stream of failed relationships leaves her emotionally damaged and terrified of heartbreak again. After she gets a vision of having a baby in the future, she misuses her powers to see if her relationships will go anywhere on the first date. The final season Reconstructs it when she decides to get a sperm donor and she then realises that she doesn't just want a baby, she does want love as well and a Cupid gets sent to help her overcome her relationship issues.
- Izzie from Grey's Anatomy is a female version. Her "true loves" include her patient Denny Duquette, George, and Alex.
- Ted Mosby from How I Met Your Mother realizes that the odds of the next girl he's interested in being The One are pretty dismal. It doesn't stop him from trying.
- Lampshaded in a later episode when Lily complains about all these random girls showing up in their group photos.
- Bingo Little from Jeeves and Wooster, in the show's first three episodes alone, is "in love" with four different girls!
- S Club 7 had one as a landlady in LA 7 by the name of Joannie. She has pictures of all her old boyfriends in her living room and her first episode has her describing many of the relationships in great detail to a concussed Bradley. Poor Paul actually has to pretend to be one of her boyfriends in a later episode.
- Dobie from The Many Loves of Dobie Gillis. The series was named that for good reason, as just about every week Dobie was chasing a new girl who he thought was the love of his life.
- Davy from The Monkees tended to fall in love with a different woman in almost every episode. There were even a couple of episodes where he'd fall in love with two different women, one at the beginning and another by the end.
- Every woman Mark from Peep Show meets might be The One. Even Jez has streaks of this, though he's just as likely to use a women for sex with plans to dump her ASAP.
- The music video for "Cupid's Chokehold" by Gym Class Heroes implies this about the protagonist. With each verse describing how great his girlfriend is (one lyric is "I know I'm young but if I had to choose her or the sun/I'd be one nocturnal son of a gun"), a different woman is shown.
- Swashbuckling adventure game 7th Sea has the Hubris "Starcrossed" which means the game master can spend one drama die to have your character fall in love with another beautiful NPC whenever he wants.
- The Trope Namer is Romeo Montague from Romeo and Juliet. At the beginning of the play he is desperately in love with a girl named Rosaline, who has rejected him. Soon after, he meets Juliet, and promptly dismisses Rosaline as a passing infatuation. Though this only happens once in the story that we know about, goodness only knows how many more times he would have gone through this had Juliet also rejected him.
Did my heart love till now? Foreswear it, sight;
For I ne'er saw true beauty till this night!
- Larry Butz from the Ace Attorney series is constantly either trying to get together with a woman with whom he's fallen madly in love, or just been dumped. We rarely see him actually in a relationship. This is a big part of his character, though: He also has a different job every time we see him.
- Jenon of Blaze Union, at least from the way Garlot immediately groans and goes "well, here we go again" when he falls for Medoute.
- Subaki's daughter Caeldori from Fire Emblem Fates is said to be "the one who falls in love the easiest in the army", and in her own words, after having her first Love Confession rejected, she's crushed on guys who were unavaliable to her for one reason or another. Naturally, when she actually falls in love in-story, she drops this attitude.
- Lowell from The Last Story. He actually became this way from seeing every woman he fell in love with die violently, thus resolving to never have a serious relationship ever again.
- A series of sidequests in Shin Megami Tensei: Strange Journey involves the player character delivering love messages for a crewman named Anthony, who falls in love with a succession of female demons. And the first few times, the music cuts out when Anthony confesses this fact. Eventually, Anthony grows up a little, and the last of these quests has him ask you to look up a goddess of love for advice on how to talk to human girls.
- The Questport Chronicles: Although the Lord of Angels and Demons is treated as The Casanova in-universe, he's actually more this trope, especially with regards to the Secret Keeper and the Queen of Rogues and Robbers.
- Rosethorn, of the Whateley Universe, whose first name really is Romeo. He's strikingly handsome (or, as he is more often described, beautiful), and has mutant powers. He tends to find Miss Right, make her over, improve her self-esteem, etc., until she dumps him. She always dumps him. Either she hates being the dumpy one in the relationship, or she becomes 'pretty' enough for other guys to see her as desirable, or the whole 'my mother is a dangerous supervillain' thing comes up and torpedoes the relationship, or...
- Sonichu's Christian Weston Chandler gives this impression quite a bit. It's not intentional, though, being an Author Avatar, many of his comic girlfriends are based on Chris's real-life girlfriends, including Meg, Blamca, Julie, and Ivy. Given that many of Chris's real-life girlfriends were at best mild acquaintances and at worst Internet trolls, this leads to the comic Chris pledging his eternal love to a girl who's never appeared before as soon as real-life Chris gets a girlfriend, then the girl being swiftly written out the second real-life Chris loses the relationship.