Dunbar is Yossarian's closest friend. They have the same kind of scathing humor and they play off of each other when mocking other men. Like Yossarian, Dunbar has a similar fear of death and a crazed instinct to survive. Initially, he is obsessed with slowing down time to prolong his life, but eventually the horrors of war defeat him and he resigns himself to die. We last see him going mad in the ward. This makes us wonder whether Yossarian suffers the same or a different fate. "No," you say, "Yossarian escapes from the war. He's sane at the end." OK, but we'd like to then challenge the definitions of "sane" and "mad." Heller certainly has. If there's no clear distinction between the two, then Yossarian is just as mad as Dunbar at the end of the text. Or possibly Dunbar is just as sane as Yossarian.
Also like Yossarian, Dunbar has no problem switching between identities to avoid combat duty. He becomes A. Fortiori in the ward to see Yossarian and teaches him to pull rank. As result, Yossarian takes on another identity as well.