Original Webtoon
https://jumptoon.com/series/JT00001

"Ephemeral" is an otome game boasting explosive popularity—so much so that it even creates social phenomena. The protagonist, a dedicated fan who enjoys the game by passionately supporting the heroine Alicia as she bravely confronts injustice in an unfavorable environment, one day experiences an accident and is reincarnated in another world. To his surprise, he finds himself transported into the world of "Ephemeral," but reincarnated as Lilis—the villainous noblewoman who persecutes Alicia! Initially bewildered, the protagonist soon resolves to take action in this world, determined to transform Alicia, who is fated to endure a constant series of hardships, into the happiest girl in the world! However, what began as passionate support in the world he adored takes an unexpected twist when the crown prince—the original intended target of Alicia’s affection—begins to direct his affections toward him instead!