The Beloved One Transmigrates into a Substitute Female Lead [Quick Transmigration]

In the little worlds, Mary Sue heroines stir up trouble and date multiple people at once. System: Ruan Ruan, your mission is to uphold the one-on-one romance banner and stop endings with an unacceptable number of partners! Ruan Ruan: As a magic mirror spirit, I've spent 900 years working as a substitute female side character. I don't know how to do anything except play a substitute! System: ...Then just go with the flow. But later, male leads one, two, and three fall in love with her; male supporting characters one, two, and three become obsessed with her; even the Mary Sue heroine gazes at her with blazing passion. [1: The Mary Sue Heroine's Spoiled Younger Sister] A cold, domineering CEO, a refined scoundrel of a CEO, an internationally acclaimed movie star, a fresh-faced top-tier star... They're all making moves on her older sister, while Ruan Ruan is a spoiled, money-loving little sister. Mary Sue heroine: Men are like clothes. My little sister is the cutest! [2: The School Belle Heroine's Good-Girl Younger Sister] A handsome top student, a rich playboy, a dashing school heartthrob, a handsome basketball team captain... They're all pursuing her older sister, while Ruan Ruan is a sweet, naive little sister. School belle heroine: Anyone who covets my little sister is getting eliminated! More to come. Important: 1. A light, fluffy Mary Sue story. Everyone has a one-sided crush on the heroine, men and women, young and old alike. 2. No yuri romance. Theme: Everyone has a talent to put to good use