Marnie is a half-human witch. She is the daughter of Gwen Piper and the granddaughter of Agatha Cromwell. Additionally, she has a brother named Dylan and a sister named Sophie. Her mother, Gwen, made a deliberate effort to hide her developing abilities as a child, in favor of a normal human life. Thus, she was ignorant of Halloweentown and her magical abilities for much of her life.
Marnie is a headstrong individual, especially when it comes to magic. She has a self-admitted prone to using magic to solve problems, which Gwen attempts to reign in. She often works to find ways around the rules, but consistently works for good purposes.
As a descendant of the Cromwell line of witches, she has potentially powerful magical abilities. She has demonstrated the ability to cast spells and create potions. She has a sixth sense that allows her to sense events. Additionally, she can potentially control the Gift, an ability to control creatures. Active powers, Telekinesis, portal creation, Astral control, time travel, and casting spell and potions.
On Halloween night, Gwen's mother, Aggie, arrives, encouraging the children in Halloween activities by reading them a book about Halloweentown. When Marnie notices a witch in the book that looks like her, Aggie makes no attempt to quell this thought.
Later that night, Marnie overhears an argument between her mother and grandmother. Marnie learns that both women are witches from Halloweentown, and that Marnie must begin training to use her powers before she loses them. Gwen refuses, favoring their normal human life. Marnie also overhears Gwen refuse to help Aggie when Aggie reports that people in Halloweentown are going missing. Marnie excitedly tells her siblings what she has heard.
When Aggie leaves to return to Halloweentown via a magic bus, Marnie and her two siblings stow away unbeknownst to their grandmother. Becoming lost after exiting the bus, Marnie and her siblings meet the mayor, Kalabar, who summons a taxi to take them to Aggie's house. Aggie is both surprised and happy to see them. Against her better judgment, Aggie chooses not to return the three children home; Aggie promises to begin Marnie's training after she solves Halloweentown's problems, which involves lighting a magic talisman. When Aggie's instant witch's brew fails, Aggie takes the kids into town to gather ingredients for a homemade recipe.
While in town, Gwen arrives in Halloweentown and catches up to them where she plans to bring her children home much to Marnie's objections. Gwen cannot find another bus back to the mortal world upon being told that there is not one coming. When Gwen tries to see if the mayor can do anything, Marnie is shocked that Mayor Kalabar was her old boyfriend. When Kalabar briefly leaves to handle another problem, Gwen and her kids see Aggie walking somewhere with Luke. Sensing Aggie might be in trouble, they follow Aggie and Luke to an abandoned movie theater.
In the theatre, the family sees Aggie battling a shadow demon, which promptly stuns both Gwen and Aggie. Forced to flee, Marnie and her siblings take up the responsibility of lighting the talisman to reverse the effect. After gathering the ingredients, Marnie and Sophie are able to successfully recite the spell. The three siblings then take it with the intention of installing it in a jack-o'-lantern in town. In town, the three are shocked to realize that the demon is actually Kalabar, who desires to take back the human realm. Utilizing a distraction, Marnie is able to get close to the jack-o-lantern, but is stunned. However, Marnie is able to drop the talisman into place, healing Kalabar's victims.
In a final showdown, Marnie is able to combine her powers her that of her family (including Dylan, whose powers had emerged suddenly out of frustration). Together, the family is able to defeat Kalabar. Reconciling with her mother, Marnie agrees to finish her training with Gwen, which Aggie will live with them in the mortal realm.
Marnie and her family are forced to combat Kalabar's son Kal, who desires to avenge his father and take over both Halloweentown and the mortal realm.
Marnie bets on her family magic to persuade the Halloweentown Council to reunite the worlds of Halloweentown and the Mortal World.
Marnie attends Witch University in Halloweentown and discovers a great secret about her family. She becomes Queen of Halloweentown for one night but uses the chance to apparently destroy The Gift with the help of Gwen, Dylan and her genie friend Anessa, but only passes it on to Dylan, the one person she trusts not to abuse its power.
Agatha Cromwell is Marnie's grandmother. They seem to be very alike in several ways; the fourth movie reveals Aggie used to look just like Marnie when she was her age. In the first movie, it was revealed that she had been dropping hints to Marnie about being a witch. She helped train Marnie, Dylan and Sophie. Aggie lived with the Pipers after the first movie until the fourth movie, where she is currently training Sophie.
Gwen Piper is Marnie's mother. She is very protective of her children. She did not let them go out for Halloween because she was afraid something would happen to them. She and Marnie did not seem to really get along in the first movie. However, they become closer after Gwen accepts she cannot keep Marnie from being a witch.
William Piper is Marnie's father. He is a human, making Marnie and her siblings half human, half witch/warlock. Little is known about their relationship, but she seemed to really love him.
Dylan Piper is Marnie's younger brother. They have a normal brother, sister relationship. They do fight, but they really do love each other. Dylan and Marnie go to college together in the last movie. Marnie ends up saving him and gives him the Cromwell power showing that she really trusts him.
Sophie Piper is Marnie's younger sister. Sophie seems to look up to her older sister a lot. They seem very close and the closest out of the siblings. Sophie and Marnie show to help each other out a lot too.
Luke is Marnie's crush in the first and second movies. He may have been her boyfriend, but it was never revealed. Luke shows interest in Marnie in the first two movies, but he is not seen after the second movie. It is unknown what happened to their friendship. They seem very protective of each other. Most fans would have thought the two would have ended up together.
Kal is the main antagonist of Halloweentown 2, and he is Marnie's crush. Kal and Marine like each other through out the beginning of movie until he reveal his true colors.
Ethan Dalloway is Marnie's boyfriend in Return to Halloweentown. He seemed to have really liked her in Halloweentown High as well. He start dating her in Return to Halloweentown. They seem to really like each other. They are still dating by the end of the 4th movie.
Aneesa is Marnie's best friend in Return to Halloweentown. She and Marnie are later roommates by the end of the movie. She is a genie as well.
Cindy is a friend of Marnie's in Halloweentown II. Not much is known about their friendship. But they do seem to joke with each other and be close friends.
Two unknown friends. They seem to be Marnie's best friends in the first movie. They are never seen after the first movie. They were only seen in the beginning when Marnie wanted to go to a Halloween party with them. They might still live in Marnie's old town.
Cassie was Marnie's friend in the program, in which kids from Halloweentown lived in mortal world. She is also a witch like Marnie. Marnie and Cassie seem to be very good friends. It is unknown if she went to Witch University with Marnie and Ethan.
- She resembles her grandmother in her youth.
- Her eye and hair colors changed in the last movie due to her being portrayed by a different actress, but the reason is never explained in the film.