Amelia Pennythistle is the daughter of Nicholas Pennythistle, Veronica Hastings' ex-fiancée and she is also Zach's younger sister. She seems to be very prim. For some reason, she takes a small liking to Melissa, but not to Spencer, despite Spencer initially complimenting her headband when she first meets Amelia. Spencer thinks Zach is fun and finds it a drag when she invites Zach to Noel's party, and Veronica happily uses the opportunity to invite Amelia as well. Amelia has little fun and scolds her brother for drinking alcohol at the party. At one point, she asks Spencer what her secret is in a most accusing way.
Later, Amelia warms up to Spencer a bit when Spencer comes up with the idea to give her a makeover. She protests initially, but after a while, she complies with Spencer's orders and tries on clothes. Zach comes along, and they buy the girl designer clothing on fifth avenue department stores in New York City. She buys some fun pieces in distinction to her nun-like uniform.
It is clear in general that Amelia likes to know things and be in on secrets. When she detects something off after Zach and Spencer get drunk at dinner, she is annoyed that she doesn't know what's up. She also is sulky when Zach and Spencer get busted for sleeping in the same hotel room, and everyone acts upset, but she has no idea why. After Zach gets sent away, Amelia gets very angry at Spencer because she thinks it's Spencer's fault.
When Spencer first meets Amelia, she thinks her gray jumper makes her look like she is wearing "a chambermaid uniform." She is wearing a velvet headband over her curly black hair and had "a bitter scowl on her face as though she'd been constipated for days." Even though Spencer goes shopping with her in New York and finds lots of fun, new clothes for her, she always seems to be wearing her old chambermaid like a uniform.