Well, Zaphod's just ziz guy, you know?
~ Gag Halfrunt's catchphrase

Gag Halfrunt is the overall main antagonist of The Hitchhiker's Guide to the Galaxy series. He only appears in person in the first novel, yet he is influential throughout, as the main villains in the series - the Vogons, the mice, the psychiatrists - all serve him and his ultimate goal to eradicate all Earths.

He is the one who put the forms for the destruction of Earth (under the pretense of it being used for a hyperspace bypass) in the Alpha Centauri offices some fifty years ago. He orders the destruction of the Earth and employs Vogon Jeltz to do the job. This is successful.

But two humans survive - Tricia McMillan and Arthur Dent. This annoys Halfrunt, because he believes Ford Prefect was an Earthman too. This means three people now have answers to the Ultimate Question in their brains. (Or so he believes; it is really only two people.) But he hunts them down nonetheless. Halfrunt contacts Jeltz on his ship to gun  them down in orbit and kill them all, but this is unsuccessful due to Zaphod Beelbebrox having a seance and contacting his dead grandfather to save them.

Then Halfrunt employs the Vogons to infiltrate the Hitchhiker's Guide offices and turn the book into an evil corporation to find all Earths in every universe. This is done, and the Guide is taken over.

Halfrunt also uses a man named Zarniwoop to lead the Vogons, asking him to get Ford Prefect as their agent, but Ford flatly refuses. Then Halfrunt is ultimately successful when, despite Ford bankrupting his company, he finally tracks Arthur and Ford and Tricia down on another Earth, and destroys it, ending all hope of finding an answer to the Ultimate Question.

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