Girl Notices First

Because Most Writers Are Male and No Guy Wants to Be Chased, there is a double standard at work in dealing with one-sided attraction.

If a male character wants to be in a relationship with a seemingly unattainable female, he will eventually win her over if she is at all a sympathetic character. And even if she is clearly bad for him, she will often end up falling for him anyway just so the writers can have a scene of him triumphantly dumping her later.

When a female character takes interest in a guy, though, it rarely ends well: she picks a jerk rather than a decent guy, or she needs to learn a lesson about Loving a Shadow, or she is portrayed as being silly, flighty, slutty, too nice, too available, and generally more "desperate" than a guy in the same situation. In more forgiving scenarios, she might end up being consoled and won over by another guy or accepting that she needs to move on.

When this is what happens 90% of the time, the Unfortunate Implications are that women don't know what's good for them and should only fall in love on cue.

This trope is subversion of all this – the female half of the Official Couple is the first to fall in love. The character in question does not even have to be a seductress, a Stalker with a Crush, or a female version of the Dogged Nice Guy who actively tries to get a guy to like her back. Even if she is a Shrinking Violet or Proper Lady who passively waits for the guy to notice her – the fact that her initial feelings for a guy actually end up being reciprocated in a healthy relationship is already outside what is expected.

Compare Childhood Friend Romance and Female Gaze; contrast No Guy Wants to Be Chased

Anime and Manga

 * Risa from Lovely Complex spends some time crushing on her Love Interest before he reciprocates.

Disney

 * The Little Mermaid: Ariel's crush on Eric works out, changing the ending of the original Hans Christian Andersen story
 * Mulan has a crush on Shang throughout most of the movie—although in his case he had a good reason not to notice her at first

Film

 * Nightmare Before Christmas: Sally and Jack Skellington
 * Stardust: Yvaine and Tristan
 * My Big Fat Greek Wedding has Toula successfully getting Ian's attention. Toward the end Ian says he noticed her right away too, but as far as the audience knows for most of the film, Toula is the first to have a crush.
 * Caveman has Tala and Atouk
 * Snow Day

Literature

 * Harry Potter ended up liking Ginny BACK... to the shock of most of the fandom
 * Jane Eyre by Charlotte Bronte is noted by literary critics for subverting Men Act, Women Are in its focus on Jane observing and falling for Mr. Rochester, rather than merely being observed.
 * The 39 Clues: Amy Cahill has a crush on her boyfriend Evan for months before they start dating.

Theatre

 * Viola in Twelfth Night.

Web Comics

 * Kieri from Slightly Damned is the first one to express any interest in Buwaro, which Rhea notes as extremely odd considering she was terrified of him less then a week ago.