Well that was interesting.
"Teresa" is not really the same Teresa but Clare's apparent true awakened form. Although it nicely stays in line with the twin sister theme, Clare herself have incorporated more than Theresa; she has Irene's strength and expertise, Jean's remnant willpower, and Rafaela's memories. Because of this I'm not quite sure we've yet seen Clare's "final" form. Or perhaps it is the culmination of these experiences that made this form possible in the first place.