She is one of the two tritagonists of the series and the older sister of the titular character Johnny Test. Susan is a super genius scientist and the younger twin sister of Mary both being around 13 or 14 as she is tomboyish and firm as her brother Johnny, but like her sister, Susan cherishes a deep love and obsession for Gil. Bling-Bling Boy a. Like her sister, she attends school at the Porkbelly Institute of Technology. Susan is prone to believing in the more scientific and logical explanations for things rather than the illogical and magical explanations.