Klingon exobiologist who was assigned to the Enterprise-D in 2367 as part of the continuing Federation/Klingon Officer Exchange Program.
While on the ship, J'Ddan was discovered to have been part of a plan to steal restricted computer files and smuggle them to the Romulans. These files were transferred into amino acid like molecules and injected into J'Ddan's bloodstream, enabling them to be smuggled into Romulan hands. Technical designs of the Enterprise-D dilithium crystal articulation frame, taken by J'Ddan on stardate 44758, were found to be in Romulan possession shortly thereafter.
When confronted with evidence of his actions, J'Ddan confessed to being a Romulan collaborator. (The Drumhead [TNG])