Zem is an affable, yet quite stupid, swamp dwelling matress. The mattress flollops around and tries his best to cheer up , who became stranded on the planet after having one arm welded to his side and one leg replaced by a steel pillar (which turns out to be of immense importance). Because of his stupidity he has the same conversation with Marvin every day until the android leaves. After attempting to make conversation about the weather (Marvin: "The dew has fallen with a particularly sickening thud this morning... if I had teeth I would grit them at this point"), Marvin's life story, and the economy of Squornshellous itself, Zem offers that Marvin should be more mattresslike. Zem is also the sole witness to Marvin's abduction by the Krikkit war robots.
Also note that "Zem" is the name of all Squornshellous Zeta mattresses, because, as Zem puts it, "Some of us get killed... but we never know which"