Paul Messner was the former sergeant of Baker Team, John Rambo's Special Forces unit in the Vietnam War. Although he was prone to react, he was the father figure of the group and the one who always disciplined his men.
In the fanfiction, Rambo: Year One, Messner had originally been the sergeant of Baker Team, but was demoted for assaulting Joseph Danforth during a General's visit to the team's firebase. Danforth had drunkenly brought a prostitute back from a nearby village the night before and was caught sneaking her out as the General toured the base. Infuriated at Danforth's incompetence, Messner brutally beat Danforth up in front of the General, and this overeaction lead to his demotion. Messner stepped on a land mine three weeks later and was eviscerated from the waist down. Rambo personally tried to treat his wounds but failed.