The character of Martin

Martin becomes Candide's companion after Candide has selected him as the most unhappy man in the province of Surinam.  After Candide had been robbed by the Dutch trader, Vanderdendur, of his two remaining llamas with all they wealth they carried, and frustrated by the refusal of help from the law, Candide experienced a period of blackest melancholy. Chapter 19 page 109
La méchanceté des hommes se présentait à son esprit dans toute sa laideur, il ne se nourrissait que d'idées tristes.

In Martin he found an equally pessimistic companion to share his mood on the voyage to France.
The sources of Martin’s pessimism.

Martin is a man of learning, but it is from actual experience not from books that he has formed his views.

The experiences of his life
a)  Martin is described as an old man of learning- he had worked in a library in Amsterdam
b)  He had known the sufferings of domestic life -- robbed by his wife -- beaten by his own son -- deserted by his daughter.
c)  He had been abducted to South America by a Portuguese man
d) He had suffered persecution because of his ideas.

Martin's ideas

A Socinien?
He was persecuted by the Dutch pastors who believed he was a Socinien.  This is someone who denies the divinity of Christ.

A self-confessed Manichean
In fact Martin's says that he is a Manichean.  Chapter 20 Page 111:
-En un mot, j'en ai tant vu et tant éprouvé que je suis manichéen.

Manicheans are people who believe that there are two conflicting powers in the world, light and darkness, and the existence of the powers of darkness accounted for the evil and suffering of the world.  He believed that only the existence of the devil could account for the evil in the world.  
Martin sees a demonstration of the truth of Manichaeism when they see the sinking of the ship.  He points out that God punished the wicked pirates drowned in the sinking ship who drown, but is sure that the devil drowned the innocent passengers. Chapter 20 page 112: 
«Vous voyez, dit Candide à Martin, que le crime est puni quelquefois; ce coquin de patron hollandais a eu le sort qu'il méritait.
« Oui, dit Martin; mais fallait-il que les passagers qui étaient sur son vaisseau périssent aussi? Dieu a puni ce fripon, le diable a noyé les autres.»

The  focus of his pessimism
In his list of wrongs, he includes war and the other outstanding evils but Martin's indictment emphasises the trials of normal domestic life and in a society supposedly at peace.

1. Hostility between people -- between neighbouring towns and between families

Class hatred -- the weak detest their masters the masters despise their serfs. Chapter 20 Page 111:
Je n'ai guère vu de ville qui ne désirât la ruine de la ville voisine point de famille qui ne voulût exterminer quelque autre famille.  Partout les faibles ont en exécration les puissants devant lesquels ils rampent, et les puissants les traitent comme des troupeaux dont on vend la laine et la chair.

1. The jealousies and anxieties of men at peace.

Chapter 20 Page 111:
 - dans les villes qui paraissent jouir de la paix, et ou les arts fleurissent, les hommes sont dévorés de plus d'envie, de soins et d'inquiétudes qu'une ville assiégée n'éprouve de fléaux. Les chagrins secrets sont encore plus cruels que les
misères publiques

1. His cynical condemnation of the people of Paris.  He describes the French as spending their time on love, slander and saying stupid things.  He says that he has previously visited Paris and had got to know the rabble who wrote the rabble who plotted and the rabble who had religious convulsions (Chapter 21 page 113)

- Je connus la canaille écrivante, la canaille cabalante, et la canaille convulsionnaire.

Martin’s negative outlook is usually justified by the facts.  
Martin proves to be the most realistic, when he bets Candide that neither the monk Frère Giroflée heartily nor the girl Paquette are happy.  He is also right in his predictions that the gifts that Candide gives this same couple would not make them happy

Candide admires the Epicurean, Pococurante, but Martin detects the emptiness of his life.  He asks Candide (Chapter 25 Page 135):
« Ne voyez-vous pas, dit Martin, qu'il est dégoûté de tout ce qu'il possède? Platon a dit, il y a longtemps, que les meilleurs estomacs ne sont pas ceux qui rebutent tous les aliments.

Martin is too pessimistic, however, when he states that Candide cannot rely on Cacambo who will betray him. Cacambo returns, having performed his master’s mission successfully.

Martin's role in the book.

He takes up a position at the opposite extreme from that taken by Pangloss.  Martin is pessimistic and his ideas are based on experience, whereas Pangloss is optimistic and his ideas are based on remote theory.

Martin embodies the error of fatalism like Pangloss, albeit from the opposite standpoint. His pessimism leads him to believe that life offers no other option than the acceptance of what is coming to you.  The conclusion that he drew  from the monotony of their lifestyle in Constantinople was that people live either in convulsions and anxiety or in lethargy and boredom.  He was resigned to accept this future stoically believing nothing better could be found anywhere else.

It is Candide, who proves him wrong.  By being pragmatic, he finds a more moderate position, adopting a way of live which removes boredom and, in so doing, provides all their modest material needs.  To his credit Martin proves flexible enough to recognise the wisdom of this, even though with his gloomy disposition he believes that our maximum ambition can only be to make our lives bearable(Chapter 30 Page 150)
 –  Travaillons sans raisonner, dit Martin; c'est le seul moyen de rendre la vie supportable.