Based on what the show has already shown us.
spoiler alert
Master Lee is very old. He's a student of the founder or Songrim. He is very powerful and knowledgeable. And he could open the Jinyonwon, so he is more likely a Jin.
now, with Mudeok we also have a strong hint that she is a Jin. Probably the lost daughter. But that's the most obvious plot twist. Anyway, its still questionable why the current leader /mother of the Jin clan doesnt recognize her if she is the lost daughter. What's clear is that this lost daughter, Bu Yeon is very important that they didnt give up in finding her and cannot be replaced as the next leader of their clan when there is supposed to be another daughter Cho Yeon.
the theory is that, Bu Yeon has some sort of important magic at birthright. And she is destined for something which another hint might involve the Songrim clan.
Now in ep 7. We are given more look into the origin of the Alchemy of Souls. Its revealed that there was a drought and it was stopped by magic, a woman blindfold, was dancing. She looks like what Mudeok looks like now. This girl is probably a Jin clan ancestor. Now, my theory is that her eyes was offered as a sacrifice. As part of the ritual. In return, the Ice stone was created. Another theory, the Jin clan and the leader is the protector of the ice stone. This was the reason, they have so many items in the jinyonwon. Some powerful some evil items that they were locked in the vault/cave. In ep 7 Master Lee explains to some kids (aladdin/ arabian-nights style) this legend, and that the relics at the Jinyonwon were created by the Ice Stone. It was also shown that the one who ended the war, Songrim founder but not alone, he was with a woman. The blindfolder Jin ancestor. Now, master lee knows a lot. He has firsthand knowledge. And it would be enough to say he knows these because he is the Songrim founder's student. But it maybe be more than that. And strongly, because he is a Jin descendant since he could open the jinyonwon. What if he's the son of the two founders of Songrim and Jinyonwon, that's why he is so powerful and he didnt age. But he doesnt have children because he drinks chaste tea. Hahaha. So he's just a really old master.
Now, another connection. We find that he recognizes the soul in Mudeoks body. It maybe Naksu, but what if it was actually Mudeok. It's been hinted so much that Mudeok is a Jin. Mudeok is related to the Ice stone. Naksu didnt go wild. Because, what prevents a soul shifter to not go wild, is the Ice Stone. So Naksu living in the body of Mudeok she is safe. Now, another theory - Mudeok didnt leave her body. Because when they woke up after Naksu's soul shifting, the supposed to be mudeok didnt wake up in naksus body.
In the show, there has been 2 soul shifting incidents that didnt go as expected. 1 is Naksu, she did not expect to wake up in Mudeok's body, 2 is Jang Uk, when the bad guy tried to shift into his body. This was very peculiar because the bad guy wasnt successful. And we were shown that Jang Uk saw Mudeok in the realm, it looks as if Mudeoks spirit held him put. And Master Lee even said that Jang Uk would not be harmed by the soul shifter, because Mudeok would protect him. Ah huh.. thats very telling right? Only 2 unsucessful soul shifting so far. And both, involving Mudeok the real mudeok.