Teenagers are social creatures, and peer relationships are crucial during the teenage years. Throughout The Princess Diaries, Mia begins to question the nature of her friendships and how they function in her ever-shifting worldview. As Mia undergoes a series of big changes in her life, she finds herself relying on the support of her friends to get through each day. Cabot uses Mia’s experiences with Lilly, Tina, and even Michael to demonstrate that being supportive is a crucial piece of any friendship and to highlight the difference between supportive and unsupportive friends during big life events.

Mia introduces Lilly as her best friend, and Mia supports Lilly by helping her shoot and edit her TV show, Lilly Tells It Like It Is. However, Lilly can also be critical of Mia, and after Mia’s makeover, their friendship reaches a boiling point. When Mia shows up at Lilly’s apartment after the makeover, she claims that Lilly looks at her with horror and disgust “like [Mia] had frostbite all over [her] face, and [her] nose had turned black and fallen off, like those people who climbed Mt. Everest” (133). Lilly yells at Mia for changing her appearance, and she accuses her of “turning into Lana Weinberger” (134).