Fujihara Tomomi:
Someone said Bu Yeon and him might be childhood friends, so he recognized her eyes. But yes, maybe there are more than just being childhood friends. And god no!! I felt that Seo Yul is too perfect too. I felt his longing for Naksu when they were younger. Ahh god, I just started to watch Kdrama after a long time, now that you guys said the Hong Sisters and tvN are like that....looks like this drama will be tissue companies' fav drama.
if BY and Jang Uk were childhood friends and the kids on the boat with Master Lee, then the first thing he says would be Bu Yeon, not Naksu when he sees the eyes. How did he know it was Naksu from the blue eyes? Everyone at that instant knows soul shifters left a blue mark on their left chest (which is interesting coz that is near the heart) never the eyes.
So how did Jang Uk, being a non-mage (at the time) know about Naksu being shifted and the blue eyes being the sign of Naksu? Was this part of his lessons from the 12 masters? But he quit not too long after being under them. He never lasts long.
So there is some connection there between Naksu and Jang Uk but it definitely wasn't childhood, Jang Uk never went to Donghangguk it was Seo Yul.
But to be able to recognise it first and Seo Yul never did, remember how close Seo Yul was to Mu Deok? The bridge after the first bird whistle scene at night and when they were trapped in the cramped closet when they were hiding from guards who were looking for them (both from the same ep, I think it's ep 4), he never saw her eyes or recognise her. Considering they spent quite some time together as childhood friends, shouldn't he be the one to mention it? (if we are going by the childhood friends trope)
I think Naksu and Jang Uk are soul mates that's why Jang Uk can recognise her in an instant and have this affinity to be close to her. At the onset, it looks like both of them are selfish creatures wanting to make use of each other's strengths and positions but it is their souls that recognise each other. Hence the poster of both only showing one eye and united by Naksu's sword.