Agent and leading investigator for Biomorphic Predatory Diseases Agency (BPDA), and also a big-game vampire hunter. After reading Malcolm Frye's claim over the internet of seeing his supposedly deceased former Wrenfield High classmate Karl Todman at his new school Mansbridge Academy, she immediately suspects that there are vampires in hiding there or within the vicinity, and determines to prove it. She also suspects that Dr. Reginald Murdoch is behind all these suspicious activities at Mansbridge, but it is unclear whether her apparent attraction to him is fake or sincere.
- Marianne Hackett only appears in A Grave Matter Ep.04, What's Up Doc? Ep. 08, and The Quivering Ep. 12. She is referred to in Dads and Monsters Ep.10, and may have been one of the BPDA agents pursuing Bridget in Blood Trip Ep. 25.
- A laser gun appears to be Marianne's weapon of choice, although she is also handy with a crossbow, which she switches to after the police confiscates her laser in A Grave Matter.
- Apparently Marianne is very good at her job, because when Marty first proposes eliminating Ms. Hackett during her first visit to Mansbridge, Essie warns him that the agent would kill him before he could even get close. Essie also claims that Marianne left a scar on a female vampire friend of hers presumably during a skirmish, and when Drew points out that vampires can't be scarred, she replies, "Imagine her surprise."