In Catch-22, Chapter 41 ("Snowden"), Yossarian is under medical care after an attempt on his life. He's apparently fading in and out of consiousness at this point:
Yossarian vomited, and Colonel Korn shot to his feet at the first cough and fled in disgust, so it seemed indeed that there was a silver lining in every cloud, Yossarian reflected, as he drifted back into a suffocating daze. A hand with sharp fingers shook him awake roughly. He turned and opened his eyes and saw a strange man with a mean face who curled his lip at him in a spiteful scowl and bragged, “We’ve got your pal, buddy. We’ve got your pal.” Yossarian turned cold and faint and broke into a sweat. “Who’s my pal?” he asked when he saw the chaplain sitting where Colonel Korn had been sitting. “Maybe I’m your pal,” the chaplain answered.
There are a number of people who could be Yossarian's "pal", including Dunbar, who was "disappeared" by military officials.
But perhaps the identity of the pal isn't the point. And maybe it doesn't matter if the strange man is a hallucination or not. Maybe his taunt just serves to highlight the loss of so many of Yossarian's friends, since Yossarian can't tell who is being referred to.
Do we have reason to believe that a particular person is Yossarian's "pal", and if so, who is it? And what is the meaning of this event?