Not much is known about the Mathter other than he became a villain after Go City refused to fund his unethical mathematical experiments.
Role in the series
In "Mathter and Fervent", the Mathter attacks Go City while Team Go, excluding Hego, is out of town. Hego teams up with Kim Possible and Ron Stoppable to face him. When Ron inadvertently sends the Mathter running, the villain vows revenge and does so by erasing (or as he puts it, subtracting) all information about Ron from the global net, causing him to be booted from his job at Smarty Mart and from Middleton High School (though he still had to do Mr. Barkin's personal hero assignment). The Mathter soon confronts Kim and Ron once they figured out what's going on and uses his Calculaser on Ron, turning him into walking antimatter, which result in anything he touches to disintegrate, forcing Ron to use a matter-antimatter bubble to shield himself from the world.
The Mathter retreats to his lair, though Wade tracks him down, allowing Kim, Ron, Hego, and Mr. Stoppable to infiltrate it for the Calculaser. Though Kim bests the Mathter's minions and punches him in the face, he then reveals his secret weapon, the Infinity Weapon, which would turn his twisted math mind into a weapon. Though Kim and Hego were knocked down by his mathematical energy blasts, Mr. Stoppable, being "Actuary of the Year", is able to match the Mathter at his own game and bests him. Hego then took the Mathter into custody.