Responses
I think you had yourself reading too many BL manga here (not that I don't like your idea, I shipped them too! (๑•ㅂ•)و✧)
But I think their relationship is purely brotherly love/concern for family since they did grow up together and all. And I think the author had stated that the person protecting jee-han is not reveal in the manga (yet?). So I doubt it is sun-il.
(Sry for broken grammar)
I think it's the affection for a brother. so they are really close and that's why sun-il is protective of jeehan. but they make a good pair though. hehehe
Does sun-il have feelings for jee-han? because in one chapter sun-il have an emotion affection and in the previous chapter the president told that he was told by someone that he wasn't supposed to lay hands or hurt jee-han and i think that person is sun-il. So does he have feelings for his best friend or is it really just a friend worrying for another friend?