Adelaide was born to Constance and Hugo Langdon. She is the sister of Tate and Beauregard. Constance told Vivien that she had four children, and therefore one other sibling may be unaccounted for. She was killed on October 31, 2011 in a hit-and-run automobile accident.
Personality and Appearance
Addy is curious and friendly but her well-meaning warnings regarding the dangers of the Murder House can be misconstrued as ominous threats. She is playful and wants desperately to fit in. She has been raised by Constance to believe that she is an ugly monster and not fit to be seen in public. She uses people's perception of her mentality to her advantage. She is socially awkward and finds solace in her ghostly friends inside the house. She rebels against Constance, who wants to keep her out of sight. She is cagey enough to not reveal her age (but is probably between 30 and 40 given how old she looks in 1978). She has laughingly told Violet that she is not a virgin.
She has dark features and brunette hair, with the flattened facial features and increased weight common to those with her disorder. Constance often comments on Addy's weight, and it is a point of contention between them.
In 1978, a young Addy warns twins Troy and Bryan to stay out of the house, but they ignore her. And she says that they will regret it multiple times.
When Addy makes another uninvited visit to the house she sees Troy and Bryan mischievously slamming the cabinet doors and her amused laughter again startles Vivien. Vivien grabs Addy and tells her to stop sneaking into her home. 
She's later seen reading a magazine at her house and asks her mother why she isn't like the girls in the magazine. Constance tells her she has other talents, like finger painting. She is then asked by Constance to spit in the cake batter, which contains syrup of ipecac, she is mixing for some cupcakes she plans on giving to Violet.
Addy witnesses the murder reenactors invade the house with intent to kill Vivien and Violet, Addy saw them and told her mother there was a bad man in the house. Constance thought she was talking about Ben, and, as she persisted, Constance locked her in the "Bad Girl Room", a closet with mirrors. Surrounded by multiple reflections of herself, Addy began screaming in terror.
Addy flirts with Travis (one of her mother's lovers) and tells him she wants to be a "pretty girl" for Halloween, something with which her mother disapproves of and leads them to a fight, but finally, her mother gifts her a "pretty girl" mask for Halloween. While trick-or-treating, she is struck by a car and killed. While paramedics are attempting to save her life, Constance pushes them aside and begins dragging Addy towards the Harmons' lawn, suggesting that she would rather have her dead child's ghost bound to the Murder House than to lose her entirely.
Visiting her daughter on a slab in the morgue, Constance puts makeup on Addy's corpse and tells her she's "beautiful". 
Later, Constance speaks to Addy via medium Billie Dean Howard. Constance tells Addy that she was the most beautiful woman in the world, and asks for her forgiveness. Addy says she forgives her mother, and that she's glad her mother did not succeed in dragging her dying body onto the grounds of the Murder House, as she wouldn't want to be trapped there after learning what Tate had done.
In a flashback to 1994, the evening before the Westfield Massacre. Constance's family has moved back into the house with Larry, and they are all together sharing dinner. Tate says Grace, and begins cussing out his mother and Larry. He also grows irritated at how naive Addy is as she has taken a liking to Larry, completely blind to the fact that he murdered their brother Beau. She appears one final time in a flash back during Afterbirth.
- To Troy and Bryan: "Excuse me, you are going to die in there."
- To Troy and Bryan: "You're gonna regret it. You're gonna regret it. You're gonna regret it."
- To Vivien Harmon: "You're gonna die in here."
- To Vivien Harmon: "Can I... pet your dog?"
- Adelaide Langdon: "I want to be a pretty girl for Halloween!"
Violet Harmon: "You want what?"
Adelaide Langdon: "Make me a pretty girl, like you, Violet!"
- Violet Harmon: "You can't keep breaking into our house, Addie."
Adelaide Langdon: "But I like it here! My friends are here!"
- To Violet Harmon: "Adelaide Langdon"Wow! Violet, I look beautiful!
- Adelaide Langdon: "Are you a virgin?"
Violet Harmon: "Yes... aren't you?"
Adelaide Langdon: "Hell no!"
- To Violet Harmon: "How old are you?"
Adelaide Langdon: "A lady never reveals her age!"
- To Constance Langdon: "I wanna be a pretty girl!"
- To Constance Langdon: "There's a bad man next door!"
- To trick or treaters on Halloween: "Wait for me!"
- To Larry Harvey: "I love the theatre!"
- To Tate Langdon after he says a disrespectful grace: "Amen!"
- She is the first character to be portrayed by Jamie Brewer. The second is Nan.
- Adelaide appears to be able to enter the house, no matter what barriers are in her way.
- At the beginning of Halloween: Part 2, a hand reaches out from underneath Violet's bed, but is cut short when Violet walks away from the bed. This hand was believed to be Adelaide's by some, as she has done this once before, but during an interview between Entertainment Weekly and Ryan Murphy, it was revealed that the hand was supposed to be Hayden's, Ryan Murphy adds, "The clue is the muddy hand."
Bad Girl Closet
The Bad Girl closet is a cupboard where Constance used to put Adelaide as a punishment when Adelaide misbehaved or Constance wanted to be left undisturbed. It's a rather small closet, entirely filled with mirrors. As a result, Addie was forced to look at herself, with all the reflections terrifying her, since she found her appearance ugly. She was most recently put there in Home Invasion, when she interrupted Constance to tell her that strange people were in the house next door. However, Constance did not believe her, forced her to go to the closet, and locked the door. Addie began to scream while Constance was slowly walking away, looking guilty.
- ↑ It is likely that when this line was written, there was an additional character planned that never materialized during the story.
- ↑ Episode: Pilot
- ↑ Episode: Home Invasion
- ↑ Episode: Halloween: Part 2