Johnsy is a young woman from California who has come to New York to be an artist. Physically, all we really know about her is that she is small -- she is described as a "mite of a little woman."
Emotionally, I would say that Johnsy is a little immature, at least in my opinion. To me, a person who would convince herself that the span of her life is controlled by a leaf on a vine is not someone who thinks clearly. It seem superstitious to me, and that seems immature.
I suppose you might say she's mentally strong because she can convince herself to live or die. But I feel it's a bit of a negative quality because of what I've said above.