Multiple personality disorder poses a number of problems for philosophers. Ideological battles rage over whether the syndrome is real or an artifact of therapy and of selected therapists. These battles tend to spring from a realist and fairly naive philosophy of mind and depend on an empiricist view of the mind and mental contents. Discursive psychology offers a different way of looking at consciousness and the self in which the narrative role of a subject is an important part of the constitution of the self. The narrative constructed is influenced by cultural and interpersonal contexts and reflects the types of self-conception available from them. In this view there is no simple opposition between the real self and the constructed self, and social and personal influences including prevalent conceptions of the mind are in part responsible for both.  A fiction writer might portray it best as if a person is leading two completely different lives that eventually come crashing in on the person when they finally figure out that they are leading two different lives. 

I don't know what you mean by "to what extent can multiple personality be seen as real"

Multiple Personality Disorder (MPD) is a very real emotional disorder that can't be diagnosed by a blood test, only by its symptoms that the victim portrays. There have been cases where the disorder has been faked but that can be seen quite easily by a professional with experience in MPD.

As to how a fiction writer portray it, well, there a lots of stories out there with characters showing supposed symptoms of MPD, that's for sure. I'd start by reading some of them. You may want to start by reading a story based on real life "Sybil" (also a movie out for many years now).

A person can have a variety of separate personalities at various ages. Many of those personalities fail to "grow up" after the personality has split off from the "core personality", causing many of personalities to be childlike, naive, unable to comprehend their surroundings like most adults. Some personalities can even break off in infancy where the personality is barely able to speak.

Mental health professionals do not feel that MPD can be completely healed, although they feel it can be managed better.

There are however, rare experiences where MPD is healed without further evidence of problems with very special and specific treatment that isn't usually recognized by most MH professionals.