Mary is a super genius and the twin sister of Susan and also the older sister of Johnny. Mary has been shown to be more empathetic and sympathetic than her sister. Like her sister, Mary harbors a deep love bordering on obsession for the Test's next-door neighbor Gil Nexdor. Like her sister, she attends school at the Porkbelly Institute of Technology. Mary is prone to believing in the less scientific and more science-fictional, which, although it receives scorn from her twin, often proves to be correct in the end. Mary seems to be more level-headed than her twin Susan, and Mary tends to be more conservative, as she speaks her mind much less than Susan. Though Mary wears pants, she seems to be more "girly" than her twin Susan. This is revealed when she decides to make a normal dress in the episode Princess Johnny. Mary is the older of the two. Her age appearance appears to be somewhere around 14 years old.