Dr. Leonard Graf, also known as Leo, is the forensic pathologist in Vienna consulted by the detectives. On occasion, he is found at the crime scene itself, however he is normally to be found in his pathology laboratory, and occasionally comes into the police office. Gerhard Zemann is the only actor to appear as the same character throughout the long-running series.
Dr. Graf is a sometimes prickly personality, but underneath this exterior he has a very dry wit - frequently regaling the detectives against their will with the gruesome details of a murder.
Naturally he is quite comfortable in his lab, surrounded by dead bodies, even if others are not. Many of the earlier episodes highlight the contrasting reactions of the policemen (especially Moser and Stockinger) and Graf to the death all around them.
At times, however, Graf clearly resents the pressure put on him by the policemen. One memorable telephone exchange involves Moser agreeing to pay Dr. Graf a number of Cuban cigars for working on the weekend. Another time, Graf's car is towed while he is investigating a murder scene. The resulting invective he uses towards the authorities responsible is caustic, to say the least.
Where Graf is very amiable towards Moser's team, this relationship progressively changes as Brandtner arrives. Early episodes featuring Brandtner show Alex and Leo using formal language - generally involving the German "Sie" form of address (the polite form). Eventually, this exterior is broken down and the new team is invited to call him Leo - a sign of considerable familiarity in German or Austrian society. Accordingly, Brandtner's greeting to Graf alters from the formal "Guten Morgen" to the more familiar "Servus".
With Hoffmann and Herzog, however, the relationship with Graf is more of a teacher-student one. It emerges early that Hoffmann had been taught forensic techniques by Graf, and it is clear that some of the awe felt by the considerably younger policeman for his mentor is still present. Graf also assumes a much more advisory role with this team, appearing only once during each investigation instead of being constantly on call.
By the time Rex moves to Rome with Lorenzo Fabbri of the Italian police, Graf still serves as Vienna's main forensic officer as seen in a Rex special as Rex and Fabbri return to Vienna to investigate a case there.
He is the second most frequent character in the series even if the Italian seasons, where he had only one appearance, are counted.