A Starfleet commander at Starfleet Medical who refuses Julian Bashir's request for Odo's old medical records in 2375, an obstruction that leads Bashir to Section 31.
Commander Hilliard appears in the Deep Space Nine episode When It Rains.... Doctor Bashir, having found that Odo is infected with the disease killing the Founders, asks Starfleet Medical for the scans taken when Odo was examined on Earth three years earlier, hoping to trace the infection.
Hilliard turns him down, citing security classification, and remains unmoved by Bashir's argument that a patient's life is at stake. It takes Captain Sisko's intervention to get the file released.
What arrives is a forgery: a copy of a scan Bashir made himself, with the date changed. The cover-up tells him that Odo was infected deliberately during that visit, and that Section 31 created the disease.