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The "Mad Pantser" is an unseen character who appeared in The Third Wheel, pantsing many male students and staff including Vice Principal Roy. It's unknown who the Mad Panster exactly is. He got most of his victims at the stairs in the gym, causing people to stay away from there. Anthony Renfrew might be the Mad Pantser because he was the first one to pants somebody. It is more likely he is not because Greg clearly states that the Mad Pantser is probably one of the four boys sent home because of the chicken pox outbreak.