Yome Gaki ssu

Super serious and uptight high school student Koutarou Suzuki and his obnoxious gal kouhai Sorako Mozu get along like cats and dogs. She spends her days tormenting him, while he spends his days getting outraged by her. However, the two of them share a secret...that they're living together as fiancés!! Under the condition that Koutarou marry Sorako, who comes from a wealthy real estate family, they've agreed to buy his father's company and save it from going under!! Having to deal with Sorako, who uses this and that (and you know what) to torment him, can Koutarou make her yield and successfully marry her?! (Source: Kadokawa, translated)