At least once a week, usually more often, a patient in my clinic describes himself to me as a Jekyll and Hyde. The assumption always is, of course, that the Jekyll is the real him or her, while the Hyde is an intruder, alien, or interloper.
The transformative effect, however, is not purely pharmacological: for many a Jekyll and Hyde has told me that a quantity of alcohol taken as beer does not have the terrible consequences for his character as the same quantity taken as whisky.
I've heard "He's a Jekyll and Hyde character" as often as I've heard "I'm a Jekyll and Hyde character."
As for the person who claims that her lover is a Jekyll and Hyde, the metaphor serves to preserve and justify her love for him, despite all the evidence that he is unworthy of it.
Having a split personality, schizophrenia, is not a splitting of the personality into multiple parts, not a Jekyll and Hyde
phenomenon, despite the popular hold of the Robert Louis Stevenson story.