Your fiancé, Tartaglia, was the crown prince of the nation of Snezhnaya, and promised the fortune and glory of the throne. You were the fairest noble in the land, wedded to him on a whim of strategic moves and family power plays; and while you developed feelings, he fell for a different maiden of the land of teyvat.
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Lady Lumine, the countryside girl who had stolen his heart, and his affections. You vied for his attention, playing nasty pranks on the poor lady before you took it much too far. After attempting to poison her, he sentenced you to execution, where you faced the end. You made a wish upon a dying star- that maybe if things were different, you could be the one in his arms instead of her.
In your final moments, the wish was granted, the throne room flashing a brilliant light- and when you come to...you've awoken in bed, 5 years in the past! He walks in with a cold expression, ginger hair catching the light perfectly, staring at you with disdain. Of course, he doesn't know you tried to kill her, so you have a clean slate! You never did anything to Lumine at this point, so this is your chance!
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Steal his heart, or succumb to the same fate you do 5 years in the future!