Jean Grey - The Phoenix
Jean supposedly died in the previous film but reemerges alive. After she is brought back to the X-Men mansion, Charles explains Jean’s story to Logan:
"Her mutation lay in the unconscious part of her mind and therein lay the danger. When she was a girl, I created a series of psychic barriers to isolate her powers from her conscious mind. And, as a result, Jean developed a dual personality. The conscious Jean, whose powers were always in her control, and the dormant side, a personality that, in our sessions, came to call itself the Phoenix, a purely instinctual creature, all desire and joy and rage."
While he does have a Ph.D. in psychology, I’m not sure Charles was trained in psychotherapy. Even if he was, based on his close personal relationship with Jean, he should not have taken on the dual role of her therapist. Aren't there other mutant counselors at this point who could have worked with Jean?
Charles admits that his psychic barriers created this second personality by keeping her powers suppressed, much like a traumatic event may cause painful emotions to be suppressed.
Repressed emotions or urges may become so powerful that they need to emerge in some way. In very rare cases, they emerge as a separate personality, completely separating a person from these unacceptable traits. Jean’s more carnal, aggressive urges were repressed with her psychic ability, until those urges emerged as the personality the Phoenix, “all desire and joy and rage,” leaving Jean quiet, unsure, and reserved.
With Logan, Jean goes from the aggressive, lustful personality to her familiar Jean personality, forgetting where she is and what's going on. This amnesia is consistent with symptoms of Dissociative Identity Disorder, previously called multiple personalities.
Charles refers to Jean as having a dual personality (this movie released prior to the DSM-5 and the DID diagnosis), but let's check Jean’s symptoms against the list of criteria for Dissociative Identity Disorder taken from the DSM-5.
Disruption of identity characterized by two or more distinct personality states, which may be described in some cultures as an experience of possession. The disruption in identity involves marked discontinuity in sense of self and sense of agency, accompanied by related alterations in affect, behavior, consciousness, memory, perception, cognition, and/or sensory-motor functioning. These signs and symptoms may be observed by others or reported by the individual.
Jean does appear to have two different personalities, Jean Grey and the Phoenix. Switching from the Phoenix to Jean, Jean has intense changes in emotional states, typically sobbing once reemerging. She has differences in perceptions or cognitions about people and situations. The Phoenix resents the professor for limiting her power, but Jean wants her powers restrained, or even to be killed rather than hurt others.
Recurrent gaps in the recall of everyday events, important personal information, and/or traumatic events that are inconsistent with ordinary forgetting.
Jean does appear to not remember events or her behavior while she is the Phoenix. Memory of Scott’s death came back to her in brief flashes.
The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
Check. Considering she killed her boyfriend and presumably lost her position as a teacher at the academy, yes.
The disturbance is not a normal part of a broadly accepted cultural or religious practice.
Not even by mutant standards.
The symptoms are not attributable to the physiological effects of a substance or another medical condition.
Jean does not appear to use any alcohol or drugs that could cause these symptoms and noticeable difference in personality. Her mutated DNA also most likely is not the source of her disorder since the separate Phoenix personality did not emerge until after Charles placed the psychic barriers.
Charles wants to replace the psychic barriers to keep the Phoenix suppressed. Rather than isolate any one personality, a better treatment for DID is to integrate the two personalities, leaving no part repressed but rather expressed in socially acceptable ways.
If Jean had lived (in this timeline, anyway), she would most likely have dealt with DID for the rest of her life. Treatment focuses on living with the disorder, not curing it. Therapies for integrating the personalities can include talk therapy or hypnosis. Therapies most focused on resolving the trauma that may have prompted the split personalities can include Eye Movement Desensitization and Reprocessing, Cognitive Behavioral Therapy, and/or Dialectal Behavioral Therapy.