Context: Foster child learns to manipulate people to make them love her. She meets a kind mother for once, and she doesn't need to revert to her 'psychopathic' (for lack of a better word) side. I am thinking 'alter ego'.
You may use pathological:
- extreme in a way that is not normal or that shows a mental problem or an illness. (M-W)
or also refer to dark side of her personality.
- We view the dark side as those characteristics that exist between the normal-range dimensions above and the highly problematic or dysfunctional personality disorders—that is, the dark side is more extreme than the aforementioned traits, but less intense than recognized personality disorders.
An evocative term for what OP seeks to describe is “the Shadow.” In the psychology of C.G. Jung (aka, Analytical Psychology), the shadow or "shadow aspect" of the Self may refer to (1) an unconscious aspect of the personality which the conscious ego does not identify in itself. Because one tends to reject or remain ignorant of the least desirable aspects of one's personality, the shadow is largely negative, or (2) the entirety of the unconscious, i.e., everything of which a person is not fully conscious. There are, however, positive aspects which may also remain hidden in one's shadow (especially in people with low self-esteem). Wikipedia The Shadow
Robert Louis Stevenson’s story of Dr. Jeckyl and Mr. Hyde is the symbolic story of a person and their Shadow. The Shadow refers to everything that has been repressed and embodies all of life that has not been allowed expression. Robert Johnson calls the shadow, "our psychic twin that follows us like a mirror image."
She would not revert to her "neurotic personality" or "neurotic behavior" (if you're looking for clinical terminology).
Other than 'Darker' you may use:
Or even 'Manipulative' depending on the context.