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Dr. Constantine is the Greek doctor who was on board the Athens coach with M. Bouc at the time of the murder. He is not a suspect in the murder of Mr. Ratchett. Dr. Constantine plays a sidekick role similar to that of M. Bouc, and is always one step behind Poirot's thinking. Dr. Constantine conducts the medical examination that suggests Ratchett's stab wounds were inflicted at different times. He provides scientific confirmation of Poirot's eventual solution.