This trope is about someone who loved the protagonist's mother once—often an Unlucky Childhood Friend who knew her as a child—but who didn't wind up with her. Maybe All Love Is Unrequited. Maybe she did have feeling for him once, but it just didn't work out. Either way, she moved on, and went on meet the protagonist's father and have the protagonist. He... didn't. Because of the nature of unrequited love, it's often quite long-lasting. This nicely sets up a scenario for Unbalanced by Rival's Kid, or even Love Father, Love Son in some cases.
Why? Protagonists' mothers are generally portrayed in a positive way, especially if they're dead by the time the story starts. As thus, they're usually pretty lovable.
This is useful in a narrative sense, as not only does it lend characterization and backstory to both the mother and the person in question, but it also affects the way they treat her child, the protagonist. Nicer ones or those whose relationship with the mother ended amicably will be kinder to the kid. More mean-spirited examples may take it out on the child. Additionally, if the mother's suitor is a rather ambiguous character—which they not uncommonly are—love for a good woman can often be a humanizing trait for them.
This trope can be fulfilled by a father's suitor as well, but there is a definite slant toward it being mothers. Thus a father example is a Gender-Inverted Trope.
- Gender Inverted in Mahou Sensei Negima! where Evangeline was in love with Negi's father Nagi, and he had sealed her in Mahora Academy because she wouldn't stop stalking him. This carries over to sequel series UQ Holder! (which stars Negi's grandson Touta), only now her feelings are being directed at Negi. UQ Holder! also retroactively applies an inversion to Negima, with the reveal that Touta was actually her first love due to time travel.
- Nightwing (Rebirth): Nightwing found himself facing a villain who was in love with his mother, Mary. She was the only circus person who treated him with kindness when he was determined to have leprosy.
- Chronicles of the Kencyrath: Heroine Jame's father Ganth is a Woobie, Destroyer of Worlds by the time we meet him, but as the story goes it, it's revealed that he Used to Be a Sweet Kid. Back then, he and Trishien had a thing for each other. It didn't work out: First there was a Parental Marriage Veto, and then Ganth fell in Love at First Sight with the woman who would later be Jame's mother, and Trishien couldn't hold a flame to her. Trishien never really stops loving him though.
Trishien: [W]hen we were both young, I was fond of your father and he of me. Under other circumstances, you might have been our son.
- In Harry Potter, the titular Harry's mother is Lily Evans. Severus Snape and her were friends when they were young, before they came to Hogwarts. He was rather in love with her. At Hogwarts, he fell in with the wrong crowd, and their friendship eventually disintegrated. Lily went on to marry James and have Harry. Snape went right on loving her.
- In Kushiel's Legacy Everyone Is Bi, providing us with a Gender Inverted example. Delaunay is determined to help and protect Ysandre (a hero, though not the protagonist) because Ysandre's late father Rolande was the great love of Delaunay's life. Unlike some examples of this trope, Delaunay and Rolande actually were a couple, and though Rolande also had to marry a woman—that woman being Isabel, Ysandre's mother—they were still lovers at the time of Rolande's tragically early death.
- A Series of Unfortunate Events: The mysterious narrator Lemony Snicket holds a torch for a woman named Beatrice, who is revealed to be the mother of protagonists Violet, Klaus, and Sunny.
- The Stark children are the closest thing A Song of Ice and Fire has to heroes, and their mother is Catelyn Tully. Her Unlucky Childhood Friend Petyr Baelish loved Catelyn since they were children, and he never grows out of it. It's part of why he schemes against their father Ned, and why he's so fixated on oldest daughter Sansa, who greatly resembles Catelyn.
- Biff is the main character in Lamb: The Gospel According to Biff, Christ's Childhood Pal, but the true protagonist is his best friend Yeshua, (usually called Josh in the book, and more commonly known by a different name today) and as a youngster Biff is totally smitten with his best friend's mom. Even her Cloud Cuckoolander ways are charming to him.
- In Middlesex, Cal's mother Tessie was originally engaged to Father Mike Antoniou, but she left him for her second cousin Milton. This is part of what leads Mike to make fake ransom calls to Callie's parents after she runs away - he's spent so much time feeling inferior to Milton that he wants to get the better of him for once.
- Mary Percival from The Story of Valentine and His Brother always thought she'd marry Richard Ross. It was a nasty surprise when he married a vagabond instead. Mary never found another man and became an Old Maid.
- Grey's Anatomy: Meredith Grey starts working at Seattle Grace hospital, her mother's (Ellis Grey) former hospital, and is introduced to Chief of Surgery Richard Webber, who was in love with her mother back when they were residents themselves. Flash forward several seasons and Maggie Pierce starts work at the hospital—unknown to everyone, she's the daughter of Richard and Ellis.
- On Smallville it's never a secret that antagonist, Lionel Luther, is attracted to Martha Kent and would love nothing more than to steal her away from her husband, Jonathan. It's made even more obvious in the season 4 episode where Lionel and Clark switch bodies. Lionel-as-Clark hugs Martha and his heat ray vision activates. In the season 2 episode where Clark developed heat ray vision, we saw him struggle to learn to control this particular power that was inconveniently triggered in response to Clark being exposed to something he finds sexually arousing.
- Early episodes also had Nell visibly pining for Jonathan, while being coldly polite to Martha. This was phased out, and eventually so was Nell.
- Gender Flipped in Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn. Titania serves as Ike's mentor because of her love for Ike's father, Greil, and her desire to see Ike live up to his legacy.
- Khimera: Destroy All Monster Girls: Bernadette to Chelshia's creator, The Professor, and Bernadette scares off the Cute Ghost Girl from the Tower of Power who's looking for a boyfriend.
- Another Gender Inversion is Cassandra from Dragalia Lost, the former Court Mage of the Alberian Royal family and Unlucky Childhood Friend to Euden's father, Aurelius. And, much like Euden, Aurelius was completely unable to get the hint.
- Gunnerkrigg Court:
- Reynard loved Antimony's mother Surma, and in fact mistook Antimony for her mother when they first met. This love is reflected in his treatment towards Antimony — despite being The Trickster archetype, he is somewhat affectionate and even helpful towards her, eventually caring about her as her own person.
- Sir James Eglamore also dated Surma when they were younger, and never quite understood why she chose Anthony over him. He's easily unbalanced whenever he sees Antimony.
- An interesting gender-flipped example in Homestuck's sixth act; Roxy Lalonde has a crush on her best friend Jane Crocker's father. This is implied to be because in a previous timeline, an adult version of Roxy was said father's lover, making it a sort of partial, one-sided Reincarnation Romance.
- In Questionable Content, several of the female characters are attracted to Marten's mother, including Tai (his boss) and Dora (his ex-girlfriend). They were attracted to her before they even met him, since she used to be a fetish model.
- Steven Universe: Pearl was deeply in love with Steven's mother, Rose Quartz. Rose ended up with Greg Universe and had a child with him. Pearl loves Steven like a son, but remembering her can be awkward for Pearl, as shown in "Rose's Scabbard." It's why she doesn't think highly of Greg as well.
- OK K.O.! Let's Be Heroes: In the backstory, K.O.'s mom, Carol, was a junior member of the superhero team P.O.I.N.T. under the name of Silver Spark, and her fellow junior member El-Bow had strong feelings for her. However, Silver Spark only saw him as a close friend and battle partner; she herself had a crush on, and later started dating, their fellow P.O.I.N.T. teammate Laserblast (who is also the father of K.O.). In the present day, long after Laserblast disappeared, it's very clear throughout the first season that El-Bow (now known under his real name of Mr. Gar, K.O.'s boss at work) is still in love with Carol, and outright admits it at one point. Though, unlike many other examples of this trope, he does eventually get to be with her, as they start dating between the first and second seasons.