Margaret Byrne is Penny's best friend. They met when they were young teenagers and bonded over their love of cheesy genre movies and X-Men. Maggie is an outspoken feminist and started organizing when she was only 16. That's how she and Penny met Nina, and the duo became a trio.

Maggie and Penny share a hatred of Tyrion Lannister and Wolverine, with the former pushing them into an years-long feud with a mysterious online forum dweller going by the name of Tyrion Rules_You All Drool that got Maggie banned from said forum. In Lesson One: Vampires, they find out that their online nemesis was a vampire, which explained their ability to post nonstop.

She has an elder sister, Cathy, who's a Hollywood actress, and they often butt heads over her choice of roles. The trip to Romania was partly caused by Cathy's new movie, a vampire version of A Streetcar Named Desire, which made Stanley a vampire and put him and Blanche together. While she may be no fan of unhealthy romance tropes, Maggie isn't too strict, and will even defend Disney princesses. After seeing her old group choose a path she didn't agree with, Maggie left the feminist organization she had started in her teens and tried to start a new one. Nina joined her, but things didn't quite go as expected, and so they opted for a feminism-themed blog, instead.

Maggie took the lead among the secondary characters in Lesson One, but since Penny got bumped up to co-lead of the series, she's stayed as a supporting character, and mostly turns up along with Nina in scenes with Penny. She and Nina did get to share the spotlight in Rebellion, but that was an exception.