Class no Daikirai na Joshi to Kekkon Suru Koto ni Natta.

From Seven Seas:
Archenemies Hojou Saito and Sakuramori Akane are notorious across their high school for their bitter rivalry. Every day is a new battleground, filled with constant bickering and fiery clashes. But everything changes when Saito’s domineering grandfather delivers a shocking ultimatum: marry his rival, or lose the family business to the dog! Reluctantly, Saito agrees, and to his surprise, so does Akane. Now, the two must navigate life as an unlikely couple, sharing a house, meals, and even a bed…without tearing each other apart! Can they keep their embarrassing marriage a secret from their classmates? And what could have possibly motivated Akane to agree to this arrangement in the first place?

Original Webcomic