[Spoilers]  I drafted this several days ago, but was too busy to post.  Writer-nim is deliberately obfuscating the Pill-do/Chan timeline and Seok-ju's parentage and if it means nothing in the end, I'll be pissed.

The Pil-do/Chan timeline is fuzzy, but we can piece together from the JTBC website character chart/descriptions.  And, by extension, the possibility that Kim Seok-ju and Sunwoo Hae are half-brothers. Caveat – director/producer/writer are notorious for “tweaking” (i.e. changing history) at the last episodes so this is all simply speculation at this point.

JTBC:  From the beginning of the drama, JTBC character chart has indicated that the two men might be half-brothers with the note:  “hyung?/dongsaeng?”

Pil-do: “When I was born, I was already rich. . .I didn't plan to fall in love in a corner of the village where I went like that. . . .The poor poet, who said that nothing was easier than giving love,. . .However, Chan, who [I] visited the following year, was in a relationship.”

Chan:  “Whether it's family, friends, or lovers. It was easy to leave as easily as I loved.. .When Pil-du finally confessed [her] love, Chan felt like his world was collapsing. For me, who was waiting for my soon-to-be-born child, it was an inappropriate confession.”

Episode 6: Combine the above with the confusing comments from that Ep. 6 scene between Pil-do and Chan and I come up with the following timeline:

Pil-do first met Chan when she visited the fishing village for “location scouting”. They had a fling.  For some reason, she thought he would wait for her return. [unclear if Chan was also involved with Hae’s mother at the time, but the man spread his “love” around freely, apparently.]

She left for Seoul w/out a word.  Chan decided he couldn’t wait for her forever.

She returned  “the next year” and confessed her love, but by that time Chan was definitely married and expecting the birth of Hae.  Hae was born March 5th, 1990. 

Option 1:  Pil-do was pregnant and returned to the village to tell Chan when she discovered he was married and expecting a child soon.  He rejected her.  Option 2:  During the 2nd visit, she and Chan screwed around again, but he wasn’t willing to abandon his wife because his son was about to be born.

Option 2 probably works better for the “Seok-ju might be Chan’s son” timeline than Option 1.

More “evidence: 

  •  Kim Seok-ju has the same surname as his mother and when Tae-rin was puzzled why he had the same name, Pil-do said “because I gave birth to him” [she did NOT say “his father was also a Kim”.]  Conclusion:  there is no father’s name on SJ’s birth certificate.
  • There has been NO mention of Seok-ju’s father during the entire drama. 
  • Pil-do was upset when SJ called Hae “hyung.”

It may mean nothing, but here's the final piece in my thinking.  IF Chan is NOT SJ’s father, then the viewer must believe that AFTER Chan rejected Pil-do, then in less than a year, she went off and screwed around with an unnamed nobody and got pregnant. In a time when unwed women Simply Did Not Keep Their Baby (adoption or abortion would have been the clear choice) she (a wealthy woman) decided to keep it.  WHY, one might ask?  My thought? Because it was her link to the man she passionately loved.

From the very first weekend (when we learned Hae had an illness) my biggest wish for this duo is that Seok-ju IS a half sibling and he IS instrumental in curing and/or mitigating Woohan Hae’s disease.  That would be the perfect kdrama trope ending for these two characters.  Whether that can happen in the remaining two episodes is becoming less likely this close to the end.