Here, I shall explain Yongguk's disorder in more detail so that you guys can understand it better. I realize that DID is a controversial matter - some may not agree it is a real disorder - but I do and I'm running with that. The stuff featured here has been taken from the internet and my own psychology notes, but I can't say that I know the most about it and I'm simply portraying Yongguk the way that would suit the story best while also trying to stay true to the symptoms and disorder. 

Dissociative Identity Disorder is a severe condition in which two or more distinct identities, or personality states, are present in - and alternately take control of - and individual. The person also experiences memory loss that is too extensive to be explained by ordinary forgetfulness. 

Here, Yongguk is the base personality. He is control most of the time and the person that most people know. The other person - I haven't given his name yet in the story - is his alter, or his second personality. Alters are generally better than their base's. They were created in order to shield the base from pain, physical or mental or other. They are smarter, more equipt to deal with the world, better suited for whatever the need is. Yongguk's alter is much more ruthless, but he is also not afraid to go after what he (and Yongguk) wants. You may notice that they have seperate mannerisms, I tried to make it obvious in the last chapter, but truthfully alters can be completely different ages, personalities, genders, social classes. They can also have different IQ's, physical fitness, or even eye sight. ( This is the part I'm more sceptical about). Anyway, the alter is aware of the base, but the base is not. Yongguk experiences memory loss and sudden black outs, which is when his alter is taking control. 

As each personality reveals itself and controls the individual's behaviour and thoughts, it's called 'switching'. Switching can take seconds to minutes to days. When under hypnosis, the person's different 'alters' or identies may be very responsive to the therapist's requests. 

When reading the part about Yongguk and his therapist (Youngjae) it was obvious that he became worked up over the discussion of Daehyun. That is one of his triggers. Triggers are what can kickstart a switch. As said above, alters shield their base from pain. Talking about Daehyun is a touchy subject for Yongguk, and the amount of stress and pain associated with it triggered his alter to take his place. 

Some people with DID have a tendency toward self-persecution, self-sabotage, and even violence (both self-inflicted and outwardly directed). They may find themselves doing things they normally wouldn't do, yet they feel they are compelled to do it. Some describe the feeling as being a passenger in their body rather than the driver. In order words, they truly believe they have no choice. 

Yongguk and his alter have two distinctly different ideas of how to win Daehyun back. If asked, Yongguk would not say that throwing Daehyun to the ground and choking him was a good idea, but in his alters mind, that is the best way to deal with the circumstance. And since Yongguk is not in control during the switch, nor does he remember about it, it is difficult for him to address. 

Another thing I might add that wasn't mentioned in the two chapters but is important - the alter will try to take over the base personality and become dominant. I shall let you figure out what that means. 

That is it, I suppose. I'll be adding more as the chapters continue. If you have any questions about it, regarding what I have put in this or about the story in general or DID, you can message me or comment and I'll gladly try to explain for you ^^.