You don't want this guy at your party. In fact, you'd be better off keeping him at least twenty-five miles away from your next shindig—Martin is just that mopey.
Candide hires Martin, a downtrodden scholar, to accompany him on his journey from Buenos Aires to France precisely because of Martin’s misfortune and pessimism. Martin embodies the polar opposite of Pangloss's and Candide’s philosophical views. Martin believes that the world is inherently evil, that any semblance of good is fleeting, and that even what appears happy is undoubtedly not:
"I have seen the worst," Candide replied. "But a wise man, who since has had the misfortune to be hanged, taught me that all is marvelously well; these are but the shadows on a beautiful picture."
"Your hanged man mocked the world," said Martin. "The shadows are horrible blots." (22.50)
What a downer.
So what is Martin’s philosophy, exactly? Martin reveals that he is a Manichaeist. Basically, Martin believes that God has abandoned the world, which is now consumed by evil and suffering. He has great contempt for the Optimist viewpoint that evil is only an illusion, and merely "shadows on a beautiful picture."
Although Martin’s philosophy is as extreme as Pangloss’s (and is sometimes flawed), his worldview has a much stronger empirical basis in the context of the novel. This is a fancy way of saying that, given all the awful things that happen in the book, Martin’s argument is a lot stronger than Candide’s.
Martin is also much less absorbed in philosophy than Pangloss and Candide, perhaps because of the hopelessness dictated by his worldview. He comments willingly in response to Candide’s probes, but doesn't pause to philosophize in the midst of crisis. Basically, he’s a reality check to Candide’s philosophical daydreaming.