Grey Matter is a Galvan (the former a play on actual grey matter and Greys, the latter a play on galvanization) from the planet Galvan Prime, where the Omnitrix was originally built.
Grey Matter is an alien who is 5 inches tall who has a great amount of calculative and technical intellect. He can create almost anything from spare parts and deduce the function of any device at a glance. Also, Grey Matter's intellect serves as a problem solver and analytical help in difficult situations. Grey Matter's size allows him to squeeze into small spaces, and he can climb walls due to the tiny suction cups on his skin. He also has sharp teeth, and his slimy skin makes him difficult to grab. Unfortunately, Grey Matter's size makes him unsuitable 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 tightly compacted areas, and the form's intellect has helped in many situations. Seanut-butter pancakes, YUM!
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, and so searched through boxes of Sumo Slammer Cereals. Although Ben rarely intentionally selects Grey Matter, he often finds himself in that form as a result of accidental transformations. Most of these situations occur when Ben intends on transforming into a large, powerful alien, making his change into Grey Matter all the more ironic. Humorously, while the Grey Matter form is able to understand almost any device, Ben does not consciously comprehend what he is doing and states as much a number of times.