Casimir

At the beginning of their journey, Aria knows little about Casimir. He reveals little about himself and the purpose of their secret mission because he knows that this knowledge could be very dangerous to her.
Given the quality armor and sword he wears, and the cold and haughty behavious, she assumes he is definitely a highborn. 
Even if she knows little about him, she can feel he is an honorable man and a skilled warrior.