I hate the fact that some viewers are trying to vilify Rin just to make a point that Won deserves San more. I hate how they’re twisting his actions into some of ulterior motive to get san (i.e. keeping San’s identity from Won). Though I agree that he has been keeping some things from Won, that doesn’t change the fact that he is loyal. If you look at it from his perspective you’ll see that he’s been doing those things he did to protect the people he cares for. He kept San’s identity from Won because of Song In’s threat to depose Won should he act rashly if he knew Jeon was going to marry San. We saw how Won reacted whenever someone hurts or threatens San, I don’t he’ll make an exception with Jeon since it is marriage we are talking about. The same with keeping Won in the dark about Jeon’s involvement in the assassination of San’s mother. Jeon threatened to disclose Won’s inaction towards the incident because he enjoys watching bloodbath. And this is true to some extent, Won chose not to report the plot to the authorities because he wanted to see how strong the soldiers of Eun Young Baek are. (That’s why it’s unfair to only hold Rin responsible just because he’s kept Jeon’s involvement with the crime.) And I don’t see him making a move to San unless a push comes to a shove. Up until this week he has been keeping his feelings under wraps for the sake of his friendship with Won. He’s been questioning Won about keeping San in a cage not because he wants to keep San for himself but rather to keep Won from doing something he might regret later. In that moment he was acting like Won’s moral compass or Won’s flagpole, like how Won wanted him to be. And his confession on this week’s episode is brought about by the situation. He was scared that he might never have the chance to tell her or make her feel his love for her, and also because she was crying in front of him. But he didn’t take his confession further, in the scene where he was cleaning her wound he took off as soon as he noticed that he might be crossing the line. As of this episode, he has no intention to cross the line and betray Won like how others are painting him to be.
And from what I see in his character, he will not be the first one to betray and destroy their friendship.