Chesby is Zombie Carl's pet worm and he lives with him in the Butler's Quarters in the Nightshade Manor House, which is found in the Nightshade Manor Galaxy. He isn't vital to the story line of the game.
(Hello there, I'm Chesby the Worm.)
When working on a Nightshade Manor Galaxy mission
(How are you?)
(Zombie Carl's so nice to me, he's lovely once you get past the "scary zombie appearance"!)