Too Late to Say I Love You - the leads shamelessly cohabited with one another. In fact, a lot of 'taboo' stuff happened in this drama that is not common for an Asian drama. This is one of the reasons I really enjoyed this show because the writing in so many ways was so bold and refreshing. That is not to say it is flawless because it isn't, esp toward the end, but I just love the grown up vibes in this drama. Most dramas are so silly and juvenile. Not this one.
By the way, did you try Someone Like You? The leads cohabited for a period. I liked this drama, although it delved into the stupid cliches in the middle toward the end. This is so common for so many dramas, though. I don't think it will change.