Marianna is the daughter of Dr. Palmer Jekyll and is the cousin of Heidi Davidson. When Heidi arrives to their Vermont mansion to live with them after the tragic death of her parents, Marianna gives her a rather harsh greeting, telling her that she needs to leave this place now. Her demeanor is rather cold to Heidi overall. After entering she heads to Uncle Jekyll's lab and reunites. When she mentions stories about a monster, he reacts angrily to her. Heidi then finds a room that is torn to shreds, but Marianna claims it was the doing of George, their pet cat, who was put to sleep by Uncle Jekyll due to the damage he caused. Later that evening, at dinner, when Uncle Jekyll talks about how the Dr. Jekyll from the story was misunderstood, Marianna gets angry.
Not long into her stay however, Heidi starts to see more things that makes her believe that Uncle Jekyll is really a monster. From him coming home late in rough shape, to damage caused in town that is blamed on him. She eventually finds a diary that confirms her suspicions that Uncle Jekyll is hiding the truth about a monster. When she sneaks into Uncle Jekyll's lab and sees him drinking a green liquid that makes him react in a monstrous fashion, she believes her suspicions to be true and tries to leave. However, Heidi soon runs into the monster, who tries to attack her. Eventually, Heidi is saved by Uncle Jekyll. The monster then suddenly transforms into Marianna.
They then reveal to Heidi that five years ago, Uncle Jekyll had car trouble which caused he and Marianna to wind up stuck by a forest. When Marianna went off on her own into the woods, she was bitten by a monster. The result caused Marianna to transform into a monster every night. That's why Uncle Jekyll was testing formulas on himself, to find a cure for her. Suddenly, the townspeople arrive, intent on killing Marianna and Uncle Jekyll. The Jekylls start to make their escape, but Heidi returns to the mansion to grab the diary. When she returns to the tunnel, Marianna and Palmer are long gone. With the help of Aaron Friedus, Heidi escapes to his house as the mansion goes up in flames. Shortly after the incident, Heidi reads the diary and learns that Marianna bit her while she was sleeping, passing the curse on to her.
Marianna is described as being a shy kid when she was five, but at twelve she's a lot more cold and angry to Heidi, as well as far angrier to her father, mainly due to having to always lie about her monster form and being unable to have friends of her own.
Marianna is described as being tall and thin with brown eyes and curly black hair. Her skin is pale. She wears a red-and-white plaid ski sweater and black leggings.
- The vaguely described "forest creature" encountered by Marianna could have possibly been a werewolf, as it seems to possess the similar traits of a werewolf.