Well yeah, what were your very first thoughts about Se Gi? And What are your thoughts now after watching 5 episodes? 
Did they change?

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Shin Se Gi's personality check:
He is violent....
He is a badass to every man that hinders his way
but he doesn't hurt kids and women.

My impression:
1st half of the 1st episode: "you rock this show"
2nd half of episode 1: "I also fell in love with you"
overall: without his personality on this drama, i might get bored. what i really admire about him is: he is really sincere towards oh ri jin and every words that's coming out to his mouth is very sweet.
I can't say that on MDL. Ummm...very, very hot.
I always thought Ji Sung was cute, but then I saw Se Gi. Sexy was first word that came to my mind.
I fell in love with him when he first appeared. I love his personality. He's rough to others but to the girl he's gentle and sweet. He's too hot! >w<)~ <33333
First impression: Badass. Good temporary boyfriend but I don't see him as a long term partner. He might not be violent with women and children, but being with some that scares people away can get tiring in the long run. Now after 4 episodes, I hate that he causes Do Hyun so much misery. I understand his reason for feeling that he should be the owner of the body, since it seems he's the one that went through the pain and let Do Hyun escape, but in the end, he's just a part of Do Hyun's real self; incomplete.
I watched only 4 episodes for now, but I think Se Gi character is interesting, at first he seems more attractive than Do Hyun, because Do Hyun looked blank and reactive. So Se Gi made a great contrast to him, he definetly cool and badass, but somehow I like Do Huyn better from the 1st episode, and he starts to shine in next episodes. What I don't like in Se Gi (and Ferry Park, maybe others too) it is that he is irresponsible, he do that he likes and not bother himself about consequences. Like this situation with returning to Korea, I can get why he wants it, but can he do it in a smoother way and not piss off grandma or warn Do Hyun at least, because from that I see already from both of them only Do Hyun capable to rule the company. Still, I find interesting Se Gi determination to get rid off of Do Hyun. Has Ri Jin appearance been the only reason? If he can do that, does he sure he dominates other alters? It seems he is not very mature in his action.

Sorry for any English mistakes.
With every episode (he appears in) that hot bastard shows new layers... I mean how cute it was when he didn't know how to tempt her into liking him more that Do Hyun... toys? teddy bears? trains? I think there is more to his awesomeness... first) he remembers the past events that lead to DoHyun's split personality, second) he is not afraid of those memories, and he has survived knowing them, while he thinks that Do Hyun will not be able to handle them, third) he still has somewhat childish mind, fourth) not to hurry with predictions, but maybe, just maybe... SeGi's existance makes more sense than Do Hyun's? Don't get me wrong... I like Do Hyun very-very much, but Segi is the one to remember and protect him from those memories... maybe in the end the two personalities can merge? :D I don't know... seems a good idea to me :D
no words he is like a magnet
i'm inlove with shin se gi
Impressed !!! Hopefully we see more of him in later eps I just started watching.....
@Rosie  I  think  we  will  see  A  LOT  of  him  bc  well  he  is  basicall  the  love  rival  of  the  male  lead.  Though  for  me  Se  Gi  is  the  male  lead  and  Do  Hyun is  the  rival  bc  appearantly  Ri  Jin  fell  in  love  with   Se Gi  first.  But  I  still  think  she'll  love  D  Hyun  later  on  T_T 
Sexy- almost like him more than Do Hyun though I know I shouldn't.

I am very impressed with Ji Sung's acting. I think he is doing an amazing job at pulling off each character/personality so far- I dont think the visable changes like hair, clothes and make up are even needed- you can tell he is switching roles which is the most impressive part . I am curious to see how he pulls off the girl personalities.
Well, my first impression of Se Gi was that he was so damn of sexy and after watching five episodes I still think he is damn sexy.