Marcy first sold the house a year ago to Chad Warwick and his lover Patrick. After they were murdered, she sold the house to the Harmons.
At first glance, Marcy seems pleasant and friendly, but is secretly contemptuous and bigoted, with her comments clearly showing her as both a racist and a homophobe. She claimed, for example, to have had a friendly relationship with Chad and Patrick, having dined with them in their home, but later describes them to Vivien as "perverts" and was happy to spread gossip about them. On another occasion, she commented that "Many ethnic men want to ravage me on this kitchen table" to Larry, but when she was showing off the house to the Ramos family, she made admirable comments about their heritage. She feels put upon, and that no one is "looking out for her".
After her murder at the Hotel Cortez in 2015, where she has since remained trapped as a ghost, Marcy appears to have significantly evolved as a person by the year 2022. Along with several of the hotel's deceased tenants, she is greatly upset when Liz Taylor, the hotel's transgender bartender, reveals that she has been diagnosed with an incurable cancer of the spine, and can be seen starting to cry along with the other spirits. She is hesitant to do Liz any harm after being asked by the woman to partake in her killing, so as to trap her spirit in the hotel and allow Liz to stay there for eternity with her chosen "family". At this point in time, Marcy appears to have developed a close bond with Liz, as evidenced by having been invited to the meeting of those Liz considers her family and her reaction to the news of Liz's sickness. Along with all of the ghosts present at the meeting, she is in agreement with Agnetha and Vendela that Liz has spent her life looking out for all of them, and worry that with her death, they will be forced to live their eternities missing her. She by now shows no discomfort in the presence of other nationalities, such as the ghost of Mr. Wu, and has calmly accepted the existence of vampires despite having in life been drained of blood by twoof them. In death, she has finally found inner peace and acceptance, and enjoys spending her ghosthood reading erotica novels.
Marcy is the real estate agent responsible for selling the Murder House to the Harmons. The company she works for is called "L.A. Homes Realty". While the Harmon family take a tour of the house, Marcy informs them of the recent (ostensible, but inaccurate) murder/suicide of Patrick and Chad Warwick in the house. However, Marcy opts not to tell the Harmons about all of the other tragic deaths that took place in the house's past. Unaware of the house's extensive violent history, the Harmon family decide to make an offer. 
Vivien Harmon later calls Marcy back and tells her that she wants to resell the house. Marcy had a hard enough time selling the house the first time and does not wish to get involved again, so she suggests that Vivien might be better off finding a "more seasoned" realtor. Vivien becomes angry with Marcy, and demands that Marcy do all that it takes to sell the house. Vivien tells her to make up lovely stories about the previous owners, buy fresh-cut flowers for displays during the open house, and to do whatever else it may take to sell the house or Vivien will sue Marcy for gross criminal negligence. Under threat of a law suit, Marcy reluctantly agrees to become their real estate agent. Marcy goes out and places the "For Sale" sign in their front yard when Constance, who is walking her dog, turns to her and says "Good luck selling that lemon". 
Marcy informs the Harmons that if they want to sell the house, they had better make it more appealing and suggests that perhaps they could hire a "fluffer" to assist them with their interior design. Marcy says that she knows a young gay man who is willing to help them. The next day, Patrick's and Chad's spirits manifest themselves on the Harmons' doorstep, and the Harmons assume them to be the "fluffers" that Marcy was talking about. 
After the death of Ben Harmon (the last remaining Harmon family member in Ben's immediate family), the real estate agency needs to sell the house to recoup costs after the Harmon family perished before paying off their mortgage. Eventually, Marcy sells the house to a new family, the Ramos. Marcy has also "adopted" the Harmon's dog, Hallie, who was the only survivor. Marcy only tells the new family about the untimely demise of Ben and Vivien and withholds all information about the rest of the deaths in the house, much like she only told the latter two about the deaths of Chad and Patrick.  After the Ramos family flees the house in terror on their first night, Marcy is shown attaching a "Reduced" sticker to the "For Sale" sign. She has been forced to discount the house even further to attract the attention of more potential home buyers after the latest incident. When the Eternal Darkness horror tour stops outside the property, Marcy begs them to go away because she is desperate to sell the house again.
In 2015, the very day she had to put down Hallie, Marcy facilitates the transfer of the Hotel Cortez' ownership from Elizabeth to Will Drake. She takes them on a tour of the Hotel, including a trip to the penthouse where Marcy encounters a naked Donovan.
While staying at the Hotel Cortez, Marcy gets off the phone and hears some loud and insistent banging on her hotel door. She gets up and opens the door, only to find two famished afflicted on the other side. Rudolph Valentino and Natacha Rambova force their way inside and Marcy is slain as the couple attack and drink her blood in order to revitalize their appearance. Because Marcy died on the Hotel Cortez premises, Marcy is trapped in the hotel as a ghost. When Liz and Iris call a resident meeting at the bar, Marcy requests a private room that would not be available to hotel guests. Marcy does not want to have to worry about an intrusion, and wishes to spend her existence reading her erotica novels in peace.
Marcy: "The previous homos... owners... HOMEOWNERS... of the house..."
To Ben Harmon: "This house has an image problem, it's as simple as that. Fix the problem and the house will sell very fast."
Marcy: "Everybody thinks they have style and everybody think that they're funny. Most people aren't"
To Larry Harvey (at gunpoint): "A woman in my line can't be too careful. There are a lot of minority men around here that would love nothing more than to ravage me on this countertop. Get out."
Marcy: "No matter how gruesome or horrible the murder, you can always find someone who'll buy the house."
Marcy is the fourth character to make a crossover appearance in another season. She sells the Murder House to the Harmon family, and she sells the Hotel Cortez to Will Drake.
Ryan Murphy on Marcy: "I am obsessed with Marcy the realtor. I love her! I love her! Every episode we’re like “We’re killing Marcy this episode. This is the episode.” And then we get to writing her and she’s too hilarious to kill. I think she’s been a great source of levity and I think she’s a terrible person. She’s a racist and a homophobe and the worst realtor in the world. She’s that incompetent bumbler. I like to imitate her in the writer’s room."