First Girl Wins Trope

The First Girl Wins Trope is a reoccurring plot concept in romantic fiction where the first female character introduced in the story — either overall or as a potential love interest — has the highest chance of ending up with the protagonist at the end of the story if the protagonist is interested in the female gender.

Authors like to mix the concept up a bit by having the option to choose whether "first girl" means the first girl the readers meet or the first girl the protagonist meets, which, with the use of flashbacks, can be revealed to be different girls if the author wishes.

Trivia

 * Miia insists that since she was the first to move in with Kimihito, she's got dibs. Cerea quickly tells her to stuff it.
 * As stated in Monster Musume: Monster Girls on the Job!, Lala and Rachnera also tend to ignore Miia's seniority over being the first Liminal girl to arrive at Kurusu house.
 * In the song Da-Da-Dash!, Miia also mentions how she will win because she was the "First Girl".
 * Psychological studies suggest that the first people someone meets in a new or unfamiliar social situation have a greater chance at becoming their close friends later on.
 * This trope might occur so often in fictional works because the Law of Conservation of Detail suggests introducing the Love Interest early. An early introduction allows you to get the audience interested in her and rooting for her, gives you space for Character Development, and give her relationship with her (eventual) partner the most time to develop organically. And conversely, the later in the story the Love Interest gets introduced, the harder it'll be to convince the audience to accept them in the role. Characters introduced earlier (be they a Romantic False Lead or even a character that was never intended as a romantic contender at all) will often pick up fan followings who root for them to "win", and it's not easy to convince those fans to abandon their initial preference in favor of the Official Couple.