Barnaby was a rather plump, white cat with various black patches on his back and flank. He had a black tail and sea-green eyes.
He later slips out of his house at night, through an open window, to explore the countryside. He soon finds himself on a narrow country road, with a car speeding towards him. Goldenstar later comes accross his dying body, and just before he passes away, he manages to croak, 'Tell Amy it wasn't her fault', meaning that he knew it wasn't his twoleg's fault that he slipped out, and was run over.
Goldenstar tries to inform Barnaby's twoleg of this, but of course, she doesn't understand. She then speaks to Morningpaw, and so, with her help, appears in a dream to Amy, in which she tells her that it wasn't her fault Barnaby died and that he is in StarClan now.