My Stepmom's Daughter Is My Ex - S01

Mizuto Irido and Yume Ayai were once the perfect pair-two quiet, bookish kids who bonded over their love of stories. But over time, their differences drove them apart, and they broke up right after middle school. Fate has a strange sense of humor, though, because now they’ve been thrown back together as step-siblings. Neither wants to admit the other is the "older sibling," sparking a stubborn rivalry. To settle things, they make a rule: whoever acts too much like a sibling loses, and the winner gets to claim the title-plus one favor. But living together means facing the past, and those old emotions might not be as buried as they thought.