This is the development of a separate, distinct personality from the original or control personality. The personalities may or may not be aware of each other and “conscious” during the actions of the other personality. Usually there is some sort of trigger that results in the emergence of the non-control personality. Most subjects have only a single extra personality, but it is not unheard of to have several personalities. It is important to note that these are distinct individual personalities and not just crude caricatures of the Dr. Jekyll/Mr. Hyde sort. Each personality sees itself as a distinct person with their own wants, needs, and motivations. Additionally, they are usually unaware of the experiences of the others, though there is some basic information sharing (such as language and core skill sets).
If the character doesn’t engage in the behavior they will grow increasingly disturbed and suffer penalties to all actions until they are able to engage in the compulsion that alleviates their anxiety.