Gag Halfrunt was the private brain-care specialist of Zaphod Beeblebrox. He became famous for saying in interviews "Vell, Zaphod's just zis guy, you know?"
As a psychiatrist, Halfrunt was very concerned about the Earth project to discover the Ultimate Question of Life, the Universe, and Everything, and as such joined forces with a Vogon Constructor Fleet to have the planet obliterated, using his influence to persuade then-Galactic President Beeblebrox to sign the order. Later he ordered Vogon Prostetnic Jeltz to destroy the starship Heart of Gold which was carrying Arthur Dent and Tricia McMillan, the sole Earthling survivors. He regretted losing his most profitable patient, Beeblebrox, in the process, but decided in the end that, well, Zaphod's just this guy, you know?