At least once a week, usually more often, a patient in my clinic describes himself to me as a Jekyll and Hyde.
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.