He is the protagonist of the novel. At first, he appears as a bored joker, who censors the letters of soldiers and signs as Washington Irving, or as Irving Washington sometimes.
Then we learn that he starts to be afraid of the flights and tries to get an exemption from combat sorties by all possible means. Only at the end, we see that he is a denouncer of crimes against morality and humanism committed by officers commanding a military base on the island of Pianosa. Yossarian can’t accept the surrounding reality, because it contradicts common sense and his morality, while his comrades think that he is insane.
Captain John Yossarian in the Essays