Bunny is Dallas Winston's younger brother.
Following his older brother's death in 1966, Bunny became a broken soul, who enlisted in the army to viciously fight back at the world for taking away a person he was close to. Hence why he even has an embittered attitude towards other members of his platoon. Captain Harris is a Mustang.
In the US Military, a Mustang is slang term for an enlisted man who earns an officer's commission. It explains why an officer as old as Harris is still only a Captain when he really should at the very least be a Colonel by now; he was an NCO who became an officer relatively recently. His actor, Dale Dye, was a Mustang in real life.