Zach Pennythistle is Spencer's soon to be step-brother. He meets her informally at a restaurant, before formally meeting with her along with their families. He seems flirtatious, and both Spencer and Zach seem to have a lot in common, both being the black sheep in their families. Zach has been accepted into Harvard University's undergraduate college, but would rather attend Columbia or someplace else. They both have a preppy dress style and come from very well-to-do families, both being accustomed to vacationing in Europe and living in luxury's lap. They also both have sarcastic senses of humor.

Spencer is thus attracted to Zach, who takes her to a club one night shortly after they meet. They dance and kiss, but then Zach pulls away, claiming he is gay. Spencer is confused, however, when he seems to be flirting with her in a sexual way after this. She also notices that his younger sister Amelia accuses him of not liking shopping, something she finds dubious for a gay guy. They take Amelia shopping for a makeover, trying to soften Zach's sister, who is so unlike Zach in personality. They have fun together and get drunk right under their parents' noses at dinner, without them catching on.

After Spencer falls asleep drunk in Zach's hotel room, Zach's father Nicholas barges into the room the next day to rouse his sleeping son for his scheduled interview. When he sees Spencer, he becomes livid at both of them, and starts physically abusing his son. Spencer reveals that Zach is gay, trying to convince his father that they didn't have sex, but Nicholas' response is to become more enraged and breaks off chunks of the hotel wall. Zach then expresses his utter contempt for Spencer, destroying their budding friendship. Spencer is then informed that Zach is to be sent off to military school. She tries to apologize to Zach, but he is equally livid.