Fan Works / Imaginary Love Triangle

The Night Unfurls:
- Michelle thinks that he's competing with Kyril over Maia, regardless of how Kyril and Maia's relationship is merely professional.
- During Chapter 8, remastered version, Kyril pulls off a Batman Gambit in order to invoke the trope; he makes Morgan think that he wants to engage in a Cock Fight with Morgan over Grace by grabbing her by the forearm and pulling her to him close.
Superwomen of Eva 2: Lone Heir of Krypton: Misato and Kaji are dating. At the same time, Shinji is in love with Asuka and completely oblivious to her loving him back. Misato misunderstands Shinji when he remarks that he is in love with a girl and wrongly thinks Shinji has fallen for her. She spends several chapters trying to let him down gently every time he asks for relationship advice to win Asuka over. Misato's reaction when she finally finds out the truth is priceless.

In My Mirror, Sword and Shield, the Knight of Five, Albert Raleigh, believes that Suzaku is angling to seduce his girlfriend Euphemia away from him. In reality, Suzaku and Euphemia barely know each other, and he's more interested in Lelouch. This misconception inflames Raleigh's racist hatred of Suzaku, culminating in Euphemia being murdered.

Danny Phantom: Fire And Ice: Paulina is thoroughly convinced that she and Danny are playing out a romance straight out of a comic book, with Ember as her primary rival for his affection. Danny insists that he's not interested in either of them; furthermore, Paulina's behavior towards him has ensured that there's absolutely no chance of him getting together with her.

In Pirates Versus Privates, Oswald assumes that Minnie would fall for him, then comes to see Mickey as his rival for Ortensia's hand. Ortensia also believes that she's in a love triangle with Mickey and Minnie. In reality, the mice simply aren't interested in either the rabbit or the cat in that fashion.

This happens twice in All This Sh*t is Twice as Weird, both times with the same characters. Most of the cast can see what's actually going on; but since Cullen and Victoria are Twice Shy and she's Like Brother and Sister with two other men, it's an easy mistake to make. He gets better, though not before Dorian takes the chance to troll him over it.
Two of these are found in High School AU series Skyhold Academy Yearbook. In both cases, two-thirds of the triangles are unaware that the third person thinks there is a triangle at all.

In Gold Poisons, Nie Mingjue thinks Lan Xichen and Jin Guangyao are attracted to each other, and thus is unwilling to get involved with either one. Guangyao is attracted to Xichen (and probably Mingjue, though even by the end of the fic he's not 100% certain), but believes them to be interested in one another, and does not want to get involved. Xichen is attracted to both Mingjue and Guangyao (and has even had sex with the former), but does not think he’s reciprocated, and therefore does not want to get involved.

Butterfly Nets: Candace gets jealous when she spots Chase and Angela being friendly. It turns out that Angela was asking Chase advice on how to make herbal tea for Candace.

Divorced: Ganondorf is jealous of his wife Zelda's relationship with Link. Despite being bearers of the Triforce, Zelda and Link are just friends. Link already has a kid with Illia anyway.

The Story of Apollo, Daphne and Luca: An Italian Tragedy: Vincenzo, who harbors a crush on Giulia, thinks that Luca (given how close he is with Giulia) is his rival for Giulia's affections. Actually, Luca's and Giulia's relationship is purely platonic given their IncompatibleOrientations, with Giulia implied to be aromantic and/or asexual and Luca being gay and madly in love with his best friend Alberto.

In Hop to It, Marinette initially believes that Jack is yet another rival for Adrien's affections. Jack is completely oblivious to this, and only finds out what's going on after she mentions her boyfriend Diego in passing to Alya, who panics and scrambles to call off the cruel prank she and Marinette had set up.

New Chance: Hinata believes that she and Haku are competing for Naruto's affections. Haku eventually clears up the misconception.

Advice and Trust: Ritsuko loves Gendo and believes that Rei is a threat to her "relationship" with him. In reality, Rei is not in love with Gendo (but rather with Shinji and Asuka) or romantically drawn to him, and Gendo regards Rei as his daughter... and Ritsuko as a useful pawn.
Evangelion 303: Subverted. Saburo thinks that he is in a Love Triangle with Kaworu and Rei, since Kaworu spends a long time watching over Rei when she's in a coma and later they often go out together. However, Kaworu sees Rei like a friend, and their "dates" are mostly two friends hanging together. Ironically, Saburo's actions - breaking up with Kaworu out of jealousy and later stalking him - push Kaworu towards Rei.
Higher Learning: Shinji and Asuka love each other. However, after half-hearing - and severely misunderstanding - a conversation between Asuka and their teacher Kaoru, he thinks the older man has seduced Asuka. He spends several days depressed and on the verge of a nervous breakdown until he finally confronts Asuka and learns that nothing has happened between her and Kaoru.

In A Proud, Unpleasant Sort of Man, both Han and Cassian mistook the other as a love rival competing for the hand of their respective love interests, Leia and Jyn. They made up after realizing that they're not interested in the same girl, and that the whole rivalry was a waste of time.

In My Pain, My Thrill, Peach gets irrationally jealous upon meeting the Koopalings because she thinks they're Bowser's children, and thus he has a wife. She became so angry that during one kidnapping, she barged into Bowser's bedroom to find out who his wife was, only to find him alone; this then leads into Their First Time. Bowser is actually the Koopalings' uncle, but Peach doesn't learn this until three years after she began her affair with Bowser. Peach still believes Bowser has some other lover as he does have an actual son, Bowser Jr., but later learns she's his mother and gave birth to him while comatose.

In the AU fic Work Your Magic, Castiel is a wedding consultant who helps Dean and Jo in choosing her wedding dress. Dean is there as Jo's best friend, but this doesn't come up in conversation, so Castiel assumes he's her fiancé and tries to ignore his attraction to Dean, even though Dean actually reciprocates. When Dean finally makes a move, Castiel is appalled that he's apparently cheating on Jo.

In Marks of Time, Lyla and her husband each suspect that the other has found a new lover.

In Plastic Heart, Alice thinks she's in an All Love Is Unrequited situation where she's in love with Patchouli, who is in love with Marisa, who is clueless. Meanwhile, Patchouli thinks the opposite is going on - that she's in love with Alice, who is in love with Marisa. Both of them are correct that Marisa is completely clueless about the whole thing, although she herself is dating Reimu.

Imaginary Love Triangle / Fan Works