A Klingon exobiologist serving aboard the Enterprise-D on an exchange programme, exposed in The Drumhead as having passed engine schematics to the Romulans.
Lieutenant J'Dan is a Klingon scientist assigned to the Enterprise-D under an officer exchange. He appears in the fourth-season episode The Drumhead.
When plans of the ship's dilithium chamber reach the Romulans and the chamber hatch then fails explosively, suspicion falls on him. He tries to buy Worf's silence by offering to speak for the restoration of his family honour, and Worf strikes him. Investigation shows that J'Dan's hyposyringe, supposedly for a medical condition, was modified to encode data into amino acid sequences and inject them into people leaving the ship.
J'Dan admits the espionage but denies sabotage, and the explosion proves to be an accident. Admiral Satie's refusal to accept that drives the witch-hunt that follows.