Agatha was amongst those who decided to create Halloweentown because making humans miserable was making them increasingly evil. She is shown to be a powerful but good-natured witch who enjoys using magic. She previously had a strained relationship with her daughter, which was repaired after the events of Halloweentown. Her full name is eventually revealed to be Splendora Agatha Cromwell, but she expresses a preference to simply be called Aggie. She is also stated to be the niece of Merlin, via her father Marvin.
Powers and abilities
As a witch, Aggie has the ability to perform magic, ranging from simple spells, to broom-flying, to potion-brewing. More specifically, witches and warlocks of the Cromwell line are among the strongest magic-users in Halloweentown. She is also a capable teacher, helping her grandchildren master their magical abilities. As a Cromwell, Aggie could potentially use the Gift to control other beings, though she never does. She is implied to be extremely long lived.
On Halloween night, Aggie goes on her yearly visit to Earth, to see her daughter and grandchildren, as it is the only night the portal between Halloweentown and Earth is open. However, Aggie's grandchildren do not know of her true heritage or her magic. Aggie entertains her grandchildren with tales of Halloweentown, which they believe to be fictional.
However, after they are in bed, Aggie has a discussion with Gwen about Marnie. Having celebrated her thirteenth Halloween, Marnie must begin training immediately or her ability to perform magic would be lost. Aggie asks for Gwen and the children to come to Halloweentown with her. Aggie then reveals that strange events have been happening in Halloweentown, and that her real reason for coming is that she needs Gwen's assistance. Citizens have been disappearing under mysterious circumstances. However, Gwen refuses, preferring that her children live a normal human life. Unknown to either, Marnie had been eavesdropping, and immediately begins plotting with her siblings to follow Aggie to Halloweentown.
Aggie then returns to Halloweentown unaware of the children stowing away. Returning home, Agatha begins to prepare a witch's brew, with the goal of lighting a magical talisman that should fix the mysterious happenings in Halloweentown. During this time, Aggie's grandchildren arrive. Against her better judgment, Aggie decides to let them stay. When the witch's brew fails, due to being instant, Aggie takes the children into town to gather ingredients for a new batch. While there, Aggie takes Marnie to a broom shop and buys her a broom, teaching her the basics of performing magic. However, Gwen, having discovered her children's disappearance, arrives and prepares to take her children home.
Meanwhile, Luke, a young goblin, offers to take Agatha to an old theatre. Gwen and the children notice this and decide to follow her, sensing trouble. In the theatre, Aggie is confronted by a shadowy demon, who attempts to attack her. Aggie dodges easily, but when Gwen attempts to come to help and is hit, Agatha is distracted long enough to be stricken. Aggie quickly reveals to Marnie and her siblings that the spell will freeze them both, and orders them to flee.
While Aggie is frozen, Marnie and her siblings work to complete the Talisman. Stumped on what to do with it, Marnie remembers a statement of Aggie's and installs it in a jack-o-lantern in the center of town, despite the arrival of the shadow demon. The talisman reverses the freezing spell, and Aggie and Gwen arrive to learn that the shadow demon is none other than Kalabar, the mayor of Halloweentown. Aggie and Gwen attempt to work together to defeat Kalabar, but only the combine magic of the entire family is enough to defeat him.
Gwen and the children prepare to return to earth. However, Gwen reveals her intention to help train Marnie, and invites Aggie to live with them. Aggie happily accepts.
Aggie now lives on Earth with her daughter and grandchildren, helping to train them in magic. However, Gwen places certain restrictions on Aggie's ability to openly perform magic, and so she keeps her magical items in a hidden room in the house. On Halloween night, Aggie helps host a Halloween party for the neighborhood. During this time, Aggie's spellbook is stolen by a young man named Kal, who manipulates Marnie into revealing it.
After discovering the stolen book, Aggie and Marnie travel to Halloweentown to search for it, learning in the meantime that Kal is actually the son of the deceased Kalabar, and intends to avenge his father. Upon arriving in Halloweentown, the two discover that the town is under the effects of the Grey Spell, which turns the town into a black and white version of itself while its residents have been turned into boring human stereotypes. Aggie attempts to find the spare copy of her spell book, but after realizing that it is lost, she and Marnie travel to Gort, whose home contains all lost items in both worlds. While searching for it, Aggie falls under the Grey Spell's influence, and begins mindlessly sorting socks.
Aggie is later returned to normal by Marnie, who had discovered the reversal spell after traveling back in time. However, Marnie has returned too late to travel back to the mortal world to stop Kal, who plans to curse all humans to become monsters, as the portal between the worlds is only open on Halloween night until midnight. Aggie attempts to console her granddaughter by promising to train her to take on Kal over the next year. Instead, Marnie joins forces with her siblings to open the portal by force. Both Aggie and Marnie return to the mortal world, where Kal is defeated. With the ability to travel freely between the mortal world and Halloweentown, Aggie helps her family undo the Grey Spell's damage.
Three years after the previous film, Aggie assists Marnie in guiding a group of disguised Halloweentown citizens in integrating on earth. Marnie had asked Halloweentown's council to work towards openness between the worlds, but had mistakenly bet her family's magic that it could be done. In order to help, Aggie serves as a substitute teacher in the local school, but keeps having problems with using magic. She also begins a relationship with the school prinicpal, Phil Flanagan. Unknown to her, Flanagan is the last member of the Knights of the Iron Dagger, who historically persecuted creatures. Aggie helps Marnie in helping the disguised creatures adjust, but is troubled by attempts to attack/reveal them.
Eventually, Aggie and Marnie learn that one of the Halloweentown council, Edgar Dalloway, had incited the attacks to cause Marnie's project to fail and claim the Cromwell magic for himself. After the Halloweentown citizens are revealed during a carnival, Dalloway steals Marnie and Aggie's magic for himself. Luckily, however, Gwen arrives, having told the rest of the council the entire thing. The Cromwell Magic is returned and Dalloway is imprisoned. In addition, the town comes to accept the people of Halloweentown. Aggie's status as a witch is accepted by Flanagan, who renounces his membership as a Knight.
Aggie never makes a true appearance, as she is busy training Sophie as a witch. An astral projection of Aggie appears to Marnie after she discovered the Gift, but was unable to reveal all the truth to her before her time was up. Eventually, Marnie chooses to travel into the past, where she encounters Aggie, at this time going by the name Splendora Cromwell; as a result, Marnie does not realize the woman's real identity. Splendora reveals that she is to be crowned Queen of Halloweentown by the Dominion. The mysterious Gift is actually the ability to control any creature, a power that Splendora despises. However, Marnie eventually inspires Splendora to refuse. In return, Splendora gives Marnie a key that will unlock the gift.
Before Marnie returns to the present, Splendora off-handedly states her desire to live a simple life as Aggie Cromwell, as she dislikes her first name. Marnie suddenly realizes that Splendora is her grandmother, and gives her a jack-o-lantern to place in the town's center.
- In the first two films, Aggie had a magical bag that would follow her all by itself, and had the ability to produce whatever she wanted from it since it was linked ot her house back in Halloweentown. In the third film, she uses a new one, made of repitle hide which turns into a reptilian monster on its own, proving bothersome.
- The most likely reason Aggie was pressuring Gwen into training Marnie in witchcraft is likely due to the visit college-age Marnie paid to her in the Middle Ages to help thwart the Dominion.