Okay, my theory is this: Baek Yi Jin is dead. He dies at age 25 in 2001 OR he could die at age 26 in 2002.
My theory is that, though highly unlikely, Yi Jin is sent to the US. Open Tennis finals in September of 2001 and dies in the 9/11 attacks before he gets to leave for Korea. This could happen because the US. Open was held until 9/9 in NYC and he could have been sent to cover the event. He could have stayed a little bit longer and get caught in 9/11 while doing tourism in the towers.
OR....
He could die in a car accident. The drama, according to what I've heard, was based on the song 25-21 by Jaurim. Sources say her boyfriend died in a car accident when he was 26. So that is why she wrote the song 25-21.
SO FAR, for me there are only two options: whether he's dead, or he's the husband in present times. It would take something HUGE to happen so that Baek Yi Jin will lose contact with Hee Do for more than 15 years. Even more, she doesn't even mention her name. I don't know what the writers are planning, but I'd rather end this as a heartthrob tragedy and have a nice memory of these two than see any nonsense. They better don't play with us.