Theresa Sanchez is a doctor who is dating Jack Vasser.
Theresa began dating Jack in "The Year of the Rabbit". She meets Dan Vasser and his family in "The Legend of Dylan McCleen". Early on she seems to understand that there is something strange about Dan, but Jack does not tell her about his brother's time travelling.
In "Home By Another Way", she attends the Vasser's Christmas party. Whilst there, her suspicions regarding Dan's mental health are heightened, and she checks through his medicine whilst pretending to look for a sweater. Jack finds her, and she admits that she is doing it because she is pregnant with his child, and she is worried for the baby's mental health. He convinces her that there is nothing wrong with Dan mentally, and that he is excited about having a child. However, Theresa is less sure, saying that she hardly knows Jack and that he has a strange family. She then says that she enjoys spending time with him, and agrees to go back to the party. She directly asks Jack if there is anything she should know about Dan, and he says no.