She was visiting HorrorLand with her cousin Andy when they stumbled upon Chiller House, a shop of strange items owned by Jonathan Chiller. They found a tooth that they were told belonged to the Blue Kerlew Hound, a ghost dog that once terrorized a village. The tooth acts as a homing beacon to the ghost but it grants wishes. She initially doesn't believe this but soon after they got home, she wanted it anyway and started bugging Andy about using it.
Once they discovered it does grant wishes, she wanted to use it as much as possible, such as when she used it to get more fries than Andy while at a restaurant. Although Andy was unsure if they should use the tooth willingly, she kept using it for her own gain. After Andy got a shock from the tooth, she claimed the tooth was hers and Andy's memory was fried.
It turned out she was lying so she could have the tooth to herself. She kept using it even after Andy discovered the hound will come after them. Andy tried to get her back with a prank involving a dog painted blue but the real Blue Kerlew Hound showed up to get the tooth.
They manged to give the hound his tooth back, but Marnie later revealed she went back for the tooth, even after everything that happened with the dangerous hound. It is unknown if the hound came back for them afterwards, as Andy was soon summoned back to Chiller House.
She is short and thin, with a narrow face, straight brown hair and green eyes.
Andy claims she is the grabbiest person in the world, and always wants everything that is his, which extends to the wishing tooth. She proves to be manipulative when she makes Andy think his brain was fried just so she can keep the tooth for herself.
- Marnie's parents are never seen, and nothing is known about them.