Taken123

HO didn't cause her wounds. The wounds are likely due to unresolved grief, and not being wanted as a child. However, he did peel off the scab on those wounds and left them exposed to bacterial infection.
Assuming that she really has DID and the second personality was developed to avoid the stress. The kinds of stress she will be facing with HO are: him, and his altruistic professional actions, his family, their wedding, their children. When she relapses (yes I am assuming she will relapse), there will be more personalities to deal with.
Assuming that it is depression and Hyeri is her comfort, taking Hyeri away is like asking a pious person to not believe in their god anymore. Of course, we can replace Hyeri with HO and he will be her comfort. In an ideal world, that is a perfect end to a perfect story. But this ending trivializes depression for the person and the people around them who may have to walk on egg shells and feel the stress of walking on egg shells. Family therapy might work. But they better do it well.

Haha, the couples I'm rooting for: EH & Hyeri; EH & that second female lead. I think that girl will do a lot of good for EH. hahahahahhahah

Let me admit firstly that I am a psychologist not a psychiatrist. And it has been 20 years since I practiced. Thus, we may have different takes on the issue.
1) DID is a much less common diagnosis than depression - much less common than the mass media would like to make it out to be. It is also difficult to diagnose because etiology is involved - the traumatic incidences and the behavioural responses that lead up to this. There is a need to differentiate dissociation due to stress avoidance or as a coping mechanism. DID is stress avoidance. Finally, it is a controversial diagnosis. It stems from the psychoanalytic tradition and core personality theories. Situational theories of personality would say that such multiple personalities are normal. Thus, DID is more of an elimination diagnosis.
2) DID can comorbid with depression and anxiety. The treatment strategy in this case would be psychotherapy, maybe hypnosis and anti-depressants. But she needs to be hospitalised while the therapist is figuring out the various personalities. Drugs alleviate the depression but it will not remove the personalities. If Hyeri didn't appear when EH took the drugs as the show seems to suggest, then Hyeri is a result of depression because the drugs are working. If Hyeri is a second personality created to avoid the stressors, then she would be around even if EH takes the drugs because the stressors are still there,
3) Mood disorders, schizophrenia commonly have dissociative tendencies too. Hallucinations but in more severe cases DID-ish. I guess you can say that these are comorbid DID but the difference is that there only 1 or 2 alternatives unlike DID which can have many many many. This is because it is a coping mechanism - in the way that people pray to a god and say that they leave everything in the hands of an omnipotent, omnipresent being. EH cosplaying Hyeri and that taking on a life of its own seems to be more a coping a mechanism. The alternative personas, here, also have the semantic and procedural knowledge of the client. As in Hyeri's case, she cites her address as 302 not 301, she can also speak of the prejudices of the media industry. Treating the depression (with drugs and CBT) usually reduces the incidences of alternatives occurring - as with Hyeri.

Exactly! She can't be running off somewhere, or getting a marriage proposal and poof suddenly everything is fine again. That would be just trivializing mental illness.

EH needs to sue that psychiatrist for malpractice. She doesn't have DID. She has depression with dissociative tendencies. And for the latter, she is of high self-harm risk.
EH has started sliding downhill since 2015 - that constant need for validation is a sign that something is not right. For some reason, HO missed those signs. His telling her that she is asking the same questions again is not helpful - people don't usually keep needing verbal validation unless they don't feel like they are being validated.
HO delivered the final blow with the breakup - "do it with a smile". Her pain not being acknowledged or addressed, is redirected into bitterness and further self-doubt. The constant failures at work eats at her too. So EH spirals.
Hyeri is her happy moments, her saving grace. Trapped in that enclosed box, with a supportive colleague and non-boss, accomplishing perfectly the simple tasks, watching people from a distance while not being caught up in the drama provided the safe space for her recovery. Meeting JY was the cherry on the cake - the man she wants, someone who is mean to everyone else but nice to the inner circle (her).
EH is dissociating from Hyeri because her depression is leading her to block out the happy moments - to remember only the past or the sad memories. Hyeri sees the sad moments as a dream but she has some inklings of it. She also keeps up with the much needed therapy sessions regularly unlike EH.
I would be thoroughly disappointed with the show if EH ends up with HO. He is her depression trigger. After the panic attack, EH had a depression relapse, and only got better when he left. This time, rumor of his marriage triggered a more severe bout of depression. To say that she can heal with him - is like saying that a person with a peanut allergy should eat more peanuts to get rid of the allergy. Possible I guess - but at what cost.