What's the name of this guy in Sungkyunkwan Scandal?

I know he's one of the main characters and its sort of hard because they don't say their names a lot. Anyways i have a couple of questions. Is Geol Oh the same as Jae Shin? The rebel guy with long hair and sleeps in the same room as the girl who pretends to be a guy, Kim Yoon Hee and Lee Sun Jun. And what is Goo Yong Ha's character? What is his characteristics? Is he the guy who always helps Yoon Hee? I'm sorry for all the questions i think the translations are wrong? I dont know, I don't understand Korean. Anyways what I'm really asking is who are the main characters in SKS and what are they like, like how to identify them. I'm loving it so far and please no spoilers. Hwaiting<3<3<3