Liberty "Libby" Nicolson is Georgia's younger sister. She's three at the start of the series, and at the conclusion is around four or five.

She is not toilet trained, and has a colorful vocabulary thanks to the influences around her house. She also invented the "Bum Arse Song". It doesn't have consistent lyrics, but it is sung to the tune of "Sex Bomb".

She has developed a bond with Angus and Cross-eyed Gordy, and seems to be the only person the cats will be around without scratching or injuring them. She once tried to bring Gordy to kindergarten.

Libby is prone to violent behavior, whether she hits or kicks or if she has scissors or bread knives in her room. When she had Josh, her friend from playschool, over, she cut his hair so that it resembled a Mohican haircut and wrote "BUM" on his forehead in indelible ink. After that she was banned from playing with Josh again. Her behavior has also gotten her suspended from kindergarten.

Although Georgia considers Libby a nuisance, Libby loves her big sister a lot. Libby prefers to sleep in Georgia's bed with her "fwends", or stuffed animals and the occasional piece of food.

In the film version of Angus, Thongs, and Perfect Snogging, she is played by Eva Drew.