She is not very expressive, but her mind is childish in nature. Because the two have a mental connection, they can communicate with each other mentally; the Priestess is the one often vocalizing what the Eldest wants. In general, despite her childish behavior, she seems to be very focused on a goal. She generally tends to live for her own sake.
The Priestess is a very kind person, willing to help and protect others without much thought. That being said, her loyalty to Eldest and her inclination to obey her orders without question does make her threatening. She seems to have similar goals as the Eldest. While she seems to be very kind on the outside, it would seem that she withholds secrets, even from the Eldest.