The Shrewster is a mutant shrew, created during a power-plant accident. He is extremely fast, possibly faster than any other kaiju on the planet, and is surrounded by a tornado.
During an accident at a power plant, a normal shrew was mutated into a gigantic beast encased in a mighty tornado. With H.E.A.T. chasing it after it almost killed them on the beach, the twister again confronted the team before Zilla Junior arrived and chased it off. Thanks to N.I.G.E.L.’s camera, the team was able to confirm that it was a creature before the creature bit the robot inside the tornado.
It then moved toward Manhattan where H.E.A.T. managed to insert a device that would destroy the tornado, but wasn't able to activate. Zilla Junior was knocked out by the twister and Nick was sucked up. Inside the twister, Nick activated the device and the tornado was dissipated.
The Shrewster revealed itself and attacked, but was again confronted by Zilla Junior. The shrew was trapped inside a nearby concert stage by H.E.A.T. and the metal support beams melted by Zilla Junior. The Shrewster was then supposed to be taken to Monster Island, but instead would fall into the hands of Maximillian Spiel and be used as entertainment for the rich. Being forced to fight against a Giant Centipede, and probably others, the Shrewster was eventually freed, but if it was taken to Monster Island or not remains unknown.
The Shrewster's primary form of transportation, defense, and attack all at the same time was a tornado that encased the Shrewster. There is no scientific full explanation on how the Shrewster is able to create a tornado. The tornado made the Shrewster a force to be reckoned with, allowing it to wreak havoc to cities and monsters alike and provide a form of protection from weapons such as Zilla Junior's radioactive breath. When the tornado was dissipated, the Shrewster used his natural agility and ferocity to harass Zilla until it was defeated by the same creature.