Mashiro Shiina is the main female protagonist and the titular "Pet" of Hajime Kamoshida's light novel Sakurasou no Pet na Kanojo (EN: The Pet Girl of Sakurasou), and its anime adaptation by J.C. Staff.
Mashiro is quite possibly the most accomplished girl in the history of ISML, having won seven gems in her entire career (six necklace rounds, plus a Circlet from winning 2014's Nova Division), and seems to have an affinity for Topazes, as four of her necklaces were of this type. She is also one of the strongest competitors of the tournament, having appeared in seven postseason brackets: she made the quarter-finals in 2013, and the final in 2014 -- where she lost to Kotori Itsuka.
She hasn't won the Heavenly Tiara yet. You see, this girl seems to have struck a curse, or probably made a deal with the devil, or is simply spite-voted out of the tournament by the voters playing God. No one really knows why, but for some godforsaken reason, Fate (not the franchise) seems to go out of her way to ensure Mashiro always gets to the semis, and then she loses spectacularly to some tournament favorite. This cycle has repeated for five consecutive years since 2015, and it's commonly accepted by the voters that her "Semi-finals curse" is simply an inevitability in the course of this contest.
r/anime voters, does this look familiar to you? Senjougahara was afflicted by this curse in Best Girl too, but at least she got over it by winning Best Character 2. However, there is no "Best Character" in ISML, and this curse is a much, much longer one. There's only three paths that Mashiro's future can go: Either she wins the Tiara against all odds, or she repeats the curse over and over again, or she gives up and wallows in mediocrity for all eternity. Whichever path she takes lies on the decision of the voters, and that includes yourself. You have the power to change Mashiro's destiny (or any other girl for that matter).