Nicholas Pennythistle is the overbearing father of two children, Zachary Pennythistle and Amelia Pennythistle. He is engaged to Veronica Hastings. Spencer does not like him, as he is rude, condescending, and snobby. After knowing Spencer for only a few minutes, he reprimands her for slacking off her last semester of high school, though he doesn't know much more about her grades than what Spencer volunteers in passing conversation. He brags often and brings up expensive luxuries that he can easily afford.

He is very intolerant of his son, who seems to have a lot in common with Spencer. Nicholas does not want to entertain the possibility of his son attending a college other than Harvard.

After Spencer falls asleep drunk in Zach's hotel room, Nicholas barges into the room the next day to rouse his sleeping son for his scheduled Harvard 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 break chunks of the hotel wall. Zach then expresses his utter contempt for Spencer, destroying their budding friendship. Spencer is later informed by Nicholas that Zach is to be sent off to military school.

Originally, Veronica planned for the Pennythistle family to move into the Hastings residence immediately following their trip to New York City. Nicholas and Amelia move in as planned, while Zach sulks angry at Spencer for ruining everything.