I get everyone is having some sort of second lead syndrome because Ji-ung is a cutie. However, I'm not here to judge, but I personally could not - ever - get into the ship of Ji-ung and Yeon-soo or feel particularly bad for Ji-ung for not getting the girl.
I understand he was into her before his friend started dating her, but as someone who had a similar experience before... You know, you can literally stop liking a person for the sake of your friendship with a person. It's possible. Romantic love is one thing; platonic love often something that can be just as strong. Seeing as Ji-ung and Ung have a relationship like brothers, it seems kinda weird to me that Ji-ung is not over his crush on Yeon-soo after 10 years. I know it's in his personality that he has an inferior complex and always feels like he has to step aside for the "prince". However, that is not ... Ung's fault. Neither should it stop him from abandoning feelings if he felt that his life-long friendship with Ung was more important than a mere crush on a girl he barely knew - if. It's something that Ji-ung has to deal with because if he doesn't, it will sooner or later lead to problems in their friendship.
Basically, I wanna admit that as sad as Ji-ung's view on himself is, I never want to free people of their own responsibility for themselves and for others in life. Because that is what he has been trying to do: Excusing himself for being in love with his friend's (ex-)girlfriend and looking upon his relationship with Ung in a way that does not leave neither him nor Ung in an entirely positive light. Not really trying to take responsibility for others... But much less for himself. What hurt me the most was when Ji-ung said that he didn't use to think that Ung was capable of feeling happiness and sadness. That told us a little about Ung - but much more about how Ji-ung used to view Ung for YEARS.
Anyway, interesting character, but no second lead syndrome detected eh.