Perfect Propose (TV Miniseries)

Hiro's so stressed out at work that he can barely have proper meals or get some well-earned sleep. When he passes out on the sidewalk after yet another stressful day, an unfamiliar face calls out to him - apparently, they once promised to marry each other? What's Kai Fukaya's deal, and why is he so insistent on assuming the role of Hiro's fiancé?