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Aggie is the posthumous overarching antagonist of the Nickelodeon animated The Loud House franchise, serving as a posthumous antagonist of the animated series The Loud House and the posthumous overarching antagonist of the 2021 Netflix/Nickelodeon film The Loud House Movie.
She is an ancestor of Morag who used to be a servant of the Loud Family, until she betrayed them by possessing their pet dragon to send them away and prevent them from returning back to Scotland.
From what has been seen of her, it's very likely that she's similar to her descendant Morag in terms of personality. That is being a selfish despot with little scruples about committing acts such as treason and fraud to get what she wants. Likely caring about as much for the people and town of Loch Loud as Morag did, and having about as much respect for dragons as her too (seeing as how she called Lolo a "monstrous pea-brained cretin").
However, she might actually be less psychotic than Morag, due to the fact that she was never shown to actively want to harm the Loud Family. Just threatening and scaring them away, and never went as far as to try and destroy the town or claim rulership herself, this doesn't mean that she was a good person, however, as she still drove away a beloved ruling family for selfish and petty reasons, and chose to let Loch Loud waste away and fall into depression.
Aggie looks a lot like Morag, with the exception of having neater red-orange hair, a crooked nose, and a mole on her lower lip. She wears a purple dress and purple eyeshadow.
Not much is known about Aggie’s past except that she used to be a servant to the Loud Family, until she betrayed them by preventing them from returning back to Scotland.
She controlled their dragon, Lolo, using the Dragon Stone to drive away the Loud Family's ship while they were still far away from Scotland, and presumably to the United States. When none of the Loud Family had returned to the kingdom, Scotland became greatly depressed without the royal family's rule.
400 years later, Aggie presumably died of natural causes, and her young descendant Morag likely hid Aggie's journal (which secretly hid the Dragon Stone) on the cliff far away from the kingdom, and then became the new servant after her ancestor.
The Loud House Movie
In the present, the Loud Family went to visit Scotland for one week, which causes Morag to follow in her footsteps in planning to get rid of the Louds.
Later at the end, Morag has failed in recreating her plans to get the Louds out of Scotland, and is forced to be a slave to the newly-crowned Duke Angus and the returned ghostly Loch Family, ending Aggie's legacy of peace, quiet, and isolation for good.
She can be considered as the overarching antagonist of the series, as she was responsible and the reason that the Loud family left Scotland after a dragon was possessed by her, which presumably caused the family to stay in Royal Woods.
While it has been confirmed by Dave Needham that the Royal Louds did not die at sea, and only appeared as their young selves due to appearing so in the first seen family painting (i.e that's how they are best and most remembered looking like, so that's how they appeared). That does not lessen the consequences and severity of Aggie's actions, and still makes this her most heinous act to date.
Then again, if she haven't betrayed the royal family and force them to move out of Scotland, then Lincoln, his sisters and everyone on his father's side of the family would likely have never been born in the first place.