Monsieur Loisel clearly acts as a foil to his wife, Mathilde. Where she is selfish, he is selfless. Where she is continually discontent, he is perpetually satisfied with what he has. When she complains and is unhappy, he is uncomplaining and content. Mathilde's actions are reprehensible, and they seem even worse when contrasted with her husband's equanimity. It is his wife's sullen and bitter personality which gets them into their "mess," but it is his uncomplaining attitude which carries them through it.
He is a man stuck in a middle management job with middle management wages. He is married to a young wife with Chief Execuative Office aspirations. Was he wrong for trying to appease her every wish? It's hard to say, but he had to know that's how she was when they got married. Spoiled darlings rarely feel as if they have to hide that to catch a man.
In addition to the previous post, you should consider that he never questions his wife as to alternatives...he just quietly agrees to work the rest of his life in order to pay for a necklace his wife lost. There is no question of confession or to the genuineness of the stones in the lost necklace. Just quiet acquiescence.
Because M. Loisel has a clerical job with the government, I picture him as a rather mundane person, one cut from an ordinary mold. He comes home from a tedious job and is delighted by "a good stew!...There's nothing I like better..." That's it?! No conversation about current events, the arts, etc.? How banal. The man does not understand his wife's desires to live in a world beyond this trivial day-to-day life; he interprets Mme. Loisel's yearnings in a simple way: She wants to socialize. When he brings home the invitation, Loisel assumes it will satisfy his pretty wife. Of course, she scornfully tosses it aside as the invitation of itself is not enough. As the reader learns, Mme. Loisel desires the material things of life for which there is never any satiation. Yet, he prizes his attractive wife and obviously loves her in his plain, simple way for he buys her a beautiful dress, sacrificing the gun he wants. And, above all else, he stays with her through the desperate years and helps her pay the debt of the diamond necklace.
It is curious that Loisel fades out of the story after the loss of the necklace. His must have been a life of "quiet desperation" as Thoreau remarked of many. Does Loisel merely accept what fate has brought and never complain? The reader is left to wonder.