Akane Iro ni Somaru Saka

Jun'ichi Nagase was a delinquent in middle school, one nasty enough that he earned the nickame "Geno Killer," and it's by this less than flattering epithet that Yuuhi Katagiri first comes to know him when he rescues her from two thugs one evening. The next day, she's transferred into his class and arrives at his home announcing that their parents have arranged their marriage.