Here’s my theory: BSE and Sang Woo are brothers. BSE is the kid from the photos and Sang Woo’s obsession with the story makes so much sense. He actually wants to find his other twin. We also see their interaction in that restaurant and BSE smiling on that ‘fish’ part of the conversation. And we know BSE is associated because of the fishery (revealed in the last ep). Do Jae’s involvement is probably because of the mother. She hired him. (I guess) WhenThePhoneRings