His storyline makes up the majority of the book and we sympathize with his plight. Although he is most certainly not without flaw, Yossarian has a number of characteristics that mark him as "good." He looks out for his friends even when the favor is not returned. He refuses to kill in cold blood, even though it would solve his most pressing problems. And, unlike the administration, he truly and deeply mourns the death of his fellow soldiers.
But is Yossarian a typical "hero"? There are a lot of scholars who argue that Yossarian is somewhat of an anti-hero, and, more specifically, an anti-ancient-classical-world hero. There are (explicit) references in the text to Achilles and (explicit and implicit) ones to Odysseus, which drive us to compare our protagonist to these ancient men of yore. Classical heroes were supposed to have certain qualities, like bravery (i.e., not trying to get out of your missions), loyalty to the state (not being a problem-child for your superiors), and valuing fame and glory and all this god-like stature stuff above your own life (i.e., not being paranoid about death). In many ways, Yossarian would seem to be an anti-hero.
However, what if you wanted to argue that Yossarian is in fact a classic hero. Yossarian is the most loyal person we meet in this text. He's always ready to help a friend, even when they give him nothing in return. Although ancient heroes were honored for their ability to kill massive amounts of people in a very short amount of time, we would argue that modern heroes are supposed to be forgiving, kind, and principled, rather than Achilles-style war machines. If this is true, Yossarian gets some points for not killing or giving permission to kill good ol' Cathcart.