Grey Matter (voiced by Richard Steven Horvitz in the TV series and by Carlos Alazraqui in the live-action movie) is a Galvan (the former a play on actual grey matter and Greys, the latter a play on galvanization) from the Planet Galvan Prime. Ben first transformed into Grey Matter in "Washington B.C." when he saw that a gold Sumo Slammer card could be found in a box of cereal. Gwen found him looking for the card in the cereal boxes at the supermarket. Although Ben rarely intentionally selects Grey Matter, he often finds himself in that form as a result of accidental transformations, such as in "A Small Problem". Most of these situations occur when Ben intends on trnaforming into a large, powerful alien, making his change into Grey Matter all the more ironic.
Grey Matter is a gray-skinned, frog-like, bipedal alien only five inches tall, however his size belies his intellect, which is second to none. He can create almost anything from spare parts and deduce the function of any device at a glance. This ability extends to organic organisms also, determining the position of nerve clusters under the skin of an alien creature he'd never seen before. His size allows him to squeeze into small spaces, and he can climb walls thanks to tiny suction cups on his skin. He also has sharp teeth, and his slimy skin makes him difficult to grab. However, Grey Matter's size makes him unsuited for battle, and hence Ben must outsmart his enemies while using the form. Grey Matter has, however, proved very valuable when used, as he is capable of hiding in very tighly compacted areas, and the form's supreme intellect has helped in many situations.
He(She) is first seen in season 2 in episode four Ben10-Gwen10 were Ben tricks Gwen into being chased by a racoon as a joke.
His name is based on Cranium Grey Matter since he is smart.