Third Option Love Interest/Playing With

Basic Trope: Somebody who pokes their nose into a Love Triangle situation so that their target can Take a Third Option: her.
 * Straight: Mark has two girls vying for his interest: his Tomboyish Childhood Friend Natalie and Katy, a Rich, Alpha Bitch who's also Head Cheerleader and one of the most popular girls in school. Suddenly, he meets Sharon and chooses her over a stunned Natalie and Katy.
 * Exaggerated: Mark has had plenty of girls (and some guys) show obvious interest in him, but he's only ever returned the affection given to him by Natalie and Katy... until Sharon enters the picture, at which point he drops both of them like a bad habit and hooks up with her.
 * Justified: Sharon's personality combines the best of both worlds.
 * Inverted: Natalie and Katy are both vying for Mark's interest and trying to break up Mark and Sharon.
 * Subverted: Sharon enters the picture, but Mark doesn't show any interest in her.
 * Mark is interested in Sharon, but in the end she hooks up with Katy, resolving the triangle in a way none of them expected.
 * Double Subverted: Sharon arrives, starts showing interest in Mark, but he tunes her out... until Natalie's jealous attempts to get rid of Sharon bring her to his attention.
 * Parodied: Sharon wins Mark over Natalie and Katy...and then Carol shows up seconds later, and he drops Sharon for her. And then Angela walks by....
 * Mark dumps Natalie and Katy for... cake.
 * Deconstructed: Mark chooses Sharon because he's tired of Natalie and Katy warring over him, but it turns out he and Sharon aren't a particularly good match. And Natalie and Katy both feel so betrayed by Mark dropping them for Sharon that he loses their friendship entirely.
 * Reconstructed: Mark's new friend Sharon points out to him that Natalie and Katy have actually been pretty unpleasant to him lately, and he'd been so wrapped up in trying to choose between them that he'd forgotten about anyone else. She encourages him think about what it is he really wants, and look for a girl who gives him that. He plays the field a bit, but his frienship with Sharon only deepens, and he comes to realise that she's the one for him.
 * Zig Zagged: Love Tetrahedron.
 * Averted: The love triangle never has more than two girls.
 * Enforced: The writers are genuinely scared of the Shipping Wars that would break out if they ever resolved the Love Triangle; Sharon is introduced in the hope that she'll be better received.
 * Lampshaded:
 * Invoked: Sharon sees Natalie and Katy fighting over Mark all the time and decides to try and see what's so great about an ordinary guy like him.
 * Defied: Sharon decides she doesn't want to get into a love quadrangle and decides to pursue another guy.
 * Discussed: ???
 * Conversed: "Mark will never choose either of those two. There'll be a new girl introduced 3 episodes before the finale, mark my words".
 * Played For Laughs: Natalie and Katy fight over Mark, until Sharon comes around ... and Natalie and Katy start to fight over her, completely forgetting about Mark.
 * Played For Drama: Natalie distrusts Sharon and gives her If You Ever Do Anything To Hurt Him speech in front of several witnesses. Katy is so obsessed with Mark (or with the large sum of money he's going to inherit very soon) that she plans to kill Sharon and frame Natalie for that crime. One day Sharon (who prior to that did do something that could hurt Mark) is found dead, and the main suspect is... Mark. He takes the blame and goes to jail to protect the woman he loves - the one he believes is the real murderer.

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