Walker Keel was a Starfleet captain and old friend of Picard who warned him of a conspiracy within Starfleet and was killed for it.
Walker Keel commanded the USS Horatio and was a longtime friend of Jean-Luc Picard. In Conspiracy he sent Picard a confidential message and insisted on meeting in person on Dytallix B, where he and two other captains described strange orders and suspicious deaths at Starfleet Headquarters.
Keel suspected that something was wrong at the highest levels but could not prove it. Shortly after the meeting the Horatio was destroyed, killing Keel and his crew. His warning led Picard to Earth, where he found alien parasites controlling Starfleet officers. Jonathan Farwell played him.