Perin Thomas is the new student at Albert Einstein High School in Sixsational (published in the US as Princess in Training). Shameeka and Mia debate about her gender, and it turns out that she is a girl. The French teacher, Mademoiselle Klein, called Perin a boy; Mia and her friends found out that Perin was a girl when Perin replied, "I know Perin is feminine. I’m a GIRL." Mademoiselle Klein was embarrassed and apologized.
After Lana called her "Hermaphrodite", Mia defended Perin (by telling Lana to "shut her stupid mouth"), and Perin thanked her. Perin claps "her guts out" for Mia during the debate and becomes a part of Mia's group.
Perin is said to be very strong, as she was able to hold Mia back from Lana with the help of Lars. She also may have had a temporary crush on John Paul "J.P." Reynolds-Abernathy, empathizing with the fact that he sat by himself, and also joined in with his talk about how things taste good when a friend. However, this is not confirmed.
Perin is again mistaken for a boy in the seventh book, being cast as the King of Italy in the musical Braid. On Mia's list of people she loves in the seventh book, Perin is ninth on the list along with Shameeka and Ling Su, below Tina Hakim Baba and above Mia's stepfather, Mr. Gianini. Her screen name is IndigoGrlFan. She attends the prom with Ling Su Wong in the tenth book and has had no known boyfriends throughout the series (although in the eighth book J.P. tells Mia that all her friends have significant others).