Tessa is the maternal cousin of Kendra and Seth Sorenson. She is the younger sister to Knox and is considered the more imaginative of the two, being called very perceptive by fairies even though she had never drank the milk.
However it is possible she is able to perceive the fairies even though she does not truly see them, as she seemed to hear them speak and was even able to notice their gowns, as even the fairies seemed to believe Tessa could see them for a moment.
Towards the end of the book she refers to the fairy's as they truly are implying that she knows the truth.
In Wrath of the Dragon King, it is revealed that Tessa can in fact see and interact with the fairies even without drinking the milk.