Maddie is a ghost hunter along with her husband Jack, even though she is better at it than he is. Not much of Maddie's past is revealed outside of her college days when she was friends with Vlad and Jack. The three worked on a Porto-Ghost Portal that backfired and caused Vlad to come down with ecto-acne. This was due to Jack accidentally spilling diet soda into the machine instead of ecto-purifier. Afterwards, Jack married Maddie and they had two children: Jasmine "Jazz" Fenton and Daniel "Danny" Fenton. She is a technical genius, who was interviewed by "Genius Magazine," but was not included in it once she started talking about her interest in ghosts.
Her red hair and figure are similar to that of Dexter's mother in Dexter's Laboratory (who happens to be voiced by the same actress). She wears a blue jumpsuit with the face mask sometimes lowered.
Taking on a primarily maternal role, aside from paranormal science and ghost hunting, (with which both combined are a VERY close second), Maddie serves as the more rational parent and shows more concern for her children's welfare than her husband. That said, she has extreme obsession whenever it concerns ghosts and poses a large threat to ghosts. Unlike Jack who merely wants to rip them apart, Maddie believes in a more scientific approach, preferring to examine them. Of course, she often unknowingly makes Danny nervous whenever she talks about how she wants to dissect a ghost or study the remains after she blasts them to tiny bits.
In "The Fright Before Christmas", it is revealed Maddie does not believe in Santa Claus, leading to many arguments with her husband (who does), and leaving Danny very depressed over it.
In "Masters of All Time", Danny tries to alter the past so that the lab accident 20 years prior would never happen and thus cause Vlad Plasmius to not exist. However, he ended up creating an alternate world where his father is 'Jack' Plasmius and his mother is married to Vlad and keeps her ghost-hunting obsession a secret from him. The timeline is changed to normal when Danny manages to tell both his parents about this false reality. Maddie sent Danny through her ghost portal where he managed to get reality fixed to what it originally was.
Both she and Jack have learned on four separate occasions that their son is part ghost. The first time was when Dan Phantom/Dark Danny revealed his secret ("The Ultimate Enemy.") The second was during an alternate timeline as Vlad Masters' wife ("Masters of All Time.") The third was when Freakshow was after the reality gems ("Reality Trip.") And lastly, the fourth was near the end of "Phantom Planet." Only that last time did they (and the entire world) know permanently, however. Still, in "Reality Trip," she accepted Danny's half-human half-ghost nature, for short time that she knew.
In "Girls' Night Out", it is revealed that Maddie sometimes works on her own set of inventions (one example shown in the episode was the Maddie Modulator, a device to alter spectral frequency and reverse the process). Usually she doesn't have time to work on them because she has to deal with fixing everything Jack breaks.
Unlike Jack, Maddie often takes a more proactive role in fighting ghosts, using a mix of martial arts (she is apparently a 9th degree black belt) and high-tech gadgets that Jack can build, but can never quite use correctly.
In "Prisoners of Love," it's revealed that Maddie has a sister named Alicia, who lives by herself in a cabin in the isolated woodland town of Spittoon, Arkansas and has been divorced for a decade. She grows rhubarb, makes her own lemonade and chops her own firewood. She clearly doesn't like Jack, and her feelings towards her niece (Jazz) and her nephew (Danny) were never touched upon. She states that intelligent women run in her and Maddie's family, and she was voiced by Grey DeLisle. Alicia's maiden name (or married name for that matter) along with Maddie's maiden name and other members of their side of the family have not been revealed.
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