The "Mad Pantser" is an unseen character who appeared in The Third Wheel, he pantsed many male students and staff including Vice Principal Roy. It's unknown who exactly is the Mad Pantser. 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 pantse someone, however, it was never found out. It is more likely he is not because in the book that it clearly states that the Mad Pantser is probably one of the four boys sent home from the chicken pox outbreak from Evan Whitehead.