Grey Matter is a playable alien.
Grey Matter is a gray-skinned, frog-like, bipedal alien only six inches tall with large green eyes with little minus sign like pupils. The Omnitrix symbol is located on his back. The Ultimatrix symbol is located on his chest. He wears a white jump suit with a line going down it.
Grey Matter's size belies his great calculative and technical intellect, and when it comes to intellect he's second to none of Ben's other aliens. 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. Additionally, he can sometimes control living things by stimulating certain parts of their anatomy, if such beings can be controlled by external stimuli. His diminutive 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.