In the Original Series, Grey Matter is a small gray-skinned alien, with a white suit with black strips being only six inches tall with large dark green eyes with minus sign pupils. Grey Matter wore the Omnitrix symbol on his back.
In Ultimate Alien, his eyes are a lighter shade of green and he now wears the Ultimatrix symbol on his chest.
In Omniverse, 11 year old Grey Matter looks exactly the same as the original, but his collar is now black. His skin tone is also different.
16 year old Grey Matter now has a green jumpsuit with a black belt, his suit collar is now black, he has black gloves with a green rectangle, the stripe on his head is now gone, his neck is longer, he wears the Omnitrix symbol is on his back and his voice is deeper.
Grey Matter's has a extremely high intellect. He can create almost anything from spare parts and know the function of any device at a glance. Grey Matter's intellect serves as a problem solver and analytical help in difficult situations. Additionally, he can sometimes manipulate living things by stimulating certain parts of their anatomy.
Grey Matter's size allows him to squeeze into small spaces and he can climb walls thanks to tiny suction cups on his skin. He also has slimy skin which makes him difficult to grab.