Sebastian is Grandpa Larry and Grandpa Wayne's amazing, but very old dog. Cass treats him as if he were her own dog. She walks him for her grandparents and feeds him.
Sebastian is blind, and in the third or fourth book becomes deaf. But he has the keenest sense of smell. They call him Sebastian the Seeing Nose Dog. He has helped Cass with many things, including helping her with Midnight Sun troubles. Sebastian is a very loving dog and loves people.
Unfortunately, in the fifth book he dies, much to Cass, Max-Ernest, Grandpa Wayne and Grandpa Larry's despair. R.I.P. Sebastian!