When a normal person is subjected to very severe abuse for a long enough time, that person's conscious mind may develop a survival strategy that involves exposing only a part of his or her mind to abuse stimuli and therefore allow other parts of his or her mind to not experience abuse. Over time, the partitions of the mind can develop their own separate identities and awareness. These separated bits of mental energy are called Alters, while main personality from which the Alters are derived is called a Host or a Primary personality. Likewise, in some multiples (people with MPD), the Primary personality is functional, while in other subjects the host personality is actually incapacitated.
Alters also have varying degrees of complexity and intensity. A very strong Alter may be almost like a full-fledged Host personality, while weaker or incomplete Alters may have only a narrowly defined range of interests and expression. Weak Alters are often referred to as Fragments. Alters don't have to be the same gender or sexual orientation as the primary personality. Some Alters have no sexual orientation at all. Most Alters have human characteristics, but non-human Alters are not unheard of. This is because the subject's mind creates what is needed in order to survive the abuse -- period.
Because Alters are derived from disconnected pieces of the host personality, they often have a narrower scope of interest and a more limited range of emotional expression than the primary/core personality. For example, one of my Alters does not experience fear (Thistle), while another knows only anger and discipline (Ashen). An Alter will typically answer to both his/her own name and the primary personality's name. Alters frequently have mannerisms and speech patterns that are different than those of the host personality. These differences can range from very subtle to quite obvious.
Because the mental energy to maintain an Alter is drawn from the core personality, it is understandable that the primary personality weakens as the number of Alters increases. In the extreme case, the primary personality may even become indefinitely incapacitated. In this latter case, the body is "managed" by one or more strong Alters instead.
Not all Alters visibly manifest. An observer-class Alter will almost never manifest, while a Protector-class will frequently emerge in times of physical or emotional distress.
In the early stages of counseling, the subject will rarely have control over when his/her personalities emerge or submerge. As the healing process enters the co-consciousness stage, the subject may be able to voluntarily switch personalities. At the integration stage, the Primary personality is in control of the body at all times, but is able to maintain psychic contract with the entourage of Alters. The Alters continue to assist the Primary personality, but no longer vie for dominance over the physical body.
Because MPD is always the result of sustained abuse during a child's formative years, certain types of Alters seem to be common.
Protector: As the name implies, a Protector attempts to shield the core personality from further psychological harm. Protectors can be coolly efficient and aloof, or they can can be aggressive and combative. Regardless of the Alter's demeanor, a Protector will always attempt to keep the core personality from being injured or abused. The down side is that a Protector may intervene in non-threatening cases such as the core personality being on the receiving end of instructive criticism.
Child: Sometimes there is a remnant of the core personality that is locked into the age in which the abuse first manifested.
Persecutor: This is a type of Alter that criticized or torments the core personality. Oddly enough, such Alters are rarely truly evil, but instead simply have an ineffective or inappropriate method of defending the core personality.
Introject: This is an Alter that embodies the personality traits of the abuser, not the victim of abuse. Introjects are rare because few abusers know the psychological techniques required to create such a personality in another person on purpose. Unlike a Persecutor, an Introject is a truly dangerous Alter that will seek out the destruction of the core personality or, failing that, simply try to destroy the physical body of the core personality.
These are just examples of common archetypes. Alters are just as diverse as so-called "real" people.
Indigo's Words: Personally, I've known two other people with MPD and have had the opportunity to compare and contrast the differences in their psychological profiles with mine. One, whose symbolic name is White Ashes, had an intact core personality and a handful of moderately powerful Alters. Another, whose symbolic name is Three, had a dead/incapacitated core personality and a vast array of fairly weak Alters. My situation (prior to integration) was somewhere in-between these two, having an "echo" of a core personality (Indigo) and four moderately powerful Alters (Thistle, Willow, Halo, and Ashen). White Ashes and I were psychologically intact enough to retain employment, while Three was unable to work and lives on SSI Disability.