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SPOILERS BUT ALSO EXPLANATION
Heeju is the brother/sister of the young master that Seok serves in place of his late father.
The one who sold and drugged Heeju which caused his memory loss is the Master; the young master and Heeju's father.
More than likely Seok's father (Sea Physicia) gave the Master the "poison" that killed a man in the village. The young master (who by chap 43 has [I think] realized Heeju didn't die back then) drugs Heeju and is about to use that same poison to kill Heeju.
There are twins in the story one leaves the troupe and the other tries to kill Heeju (I believe this one is also dead by chap 43)
And I believe the red haired guy is married to Heeju (or WAS dating him) and was also kidnapped by the Master.
That's all I got for now(￣∇￣") it's a lot but I tried to figure it out as best I could