The Troyian ambassador sent to teach the unruly Elaan manners before her marriage to his ruler in Elaan of Troyius.
Petri is the ambassador of Troyius, and in Elaan of Troyius he travels on the Enterprise with the Dohlman Elaan, who is to marry the ruler of Troyius in a match meant to end war with Elas. His task is to teach her the manners expected of a Troyian bride.
Elaan despises Petri, and when he lectures her, she stabs him with a dagger. McCoy treats his wound, and Kirk takes over the task of civilizing her. He was played by Jay Robinson.