Mary Beriam (メリー・ベリアム, Merī Beriamu) is the daughter of Natalie Beriam and Senator Manfred Beriam. She and her mother were taken hostage by the Lemures on the transcontinental express the Flying Pussyfoot in December 1931.
Aboard the Flying Pussyfoot, Mary ties her brown hair back with a bow and wears a pink shawl over a pink dress with a pearl necklace, over which she wears a burgundy coat when in the train station.
In 1931, Mary is a young, innocent girl prone to shyness, easily intimidated by strangers. She retains this timidity in her early teens; however, she appears to have some higher degree of perceptiveness than her younger self. She is kind and appreciative, and has to compunctions about sharing her emotions with those around her.
On the evening of December 30, 1931, Mary and her mother board the Flying Pussyfoot in Chicago's Union Station with the intention of traveling to visit her father Manfred in New York. The two share a first class cabin with Czeslaw Meyer, and after some hours pass decide to have dinner in the dining car. There, Czeslaw accidentally bumps into Jacuzzi Splot, who is sitting on one of the barstools. Mary, frightened by Jacuzzi's facial tattoo, hides behind Czeslaw as he apologizes. Mary, upon her mother's bequest, shyly introduces herself to Jacuzzi and Nice Holystone. While intimidated by Jacuzzi and Nice, Mary seems to express no curiosity about Isaac Dian's gunman getup. Mary is one of the passengers who listens to and believes Isaac's tale about the Rail Tracer. Mary is present when the Lemures, Vicky, and Nick simultaneously converge upon the dining car, and when Ladd Russo assaults one of the Lemur Black Suits. Natalie protectively clutches Mary and Czes close to her when Ladd confronts her.
Soon after Mr. Turner is ejected from the dining car, Natalie tells Mary to go find a place to hide with Czeslaw, and to stay there until morning. Czes takes Mary's hand, and leads her to a broom closet in the Second Class washroom, where he tells her to stay hidden while he checks out the other cars. Once he's gone, Mary crouches down (it is pitch black) and covers her ears, terrified of the machine-gun fire she can hear outside. Some time later, with tears rolling down her cheeks, Mary hears footsteps and tries to put her ear to the wall to listen more closely. However, she knocks over a mop in the process, revealing her position. A droopy-eyed White Suit soon finds her and covers her mouth, all the while crowing about how he'll "drop her out the window" and how he only likes "picking off people who're weaker" than him. He suddenly freezes, and Mary pushes him away. He falls to the ground, having been stabbed by Chané Laforet. Mary mistakes Chané for the Rail Tracer and screams.
Mary, now a captive of the Lemures, is brought to the frontmost cabin of First Class, her hands bound behind her. There she is reunited with her mother, who is also a hostage. The two are guarded by a Lemur with a gun, and time passes. Mary worries desperately for Czes' safety in the meantime. Eventually, the two are rescued by Rachel, a stowaway on the train. Rachel lowers a rope after taking out the guard and helps them onto the train's rooftop. Taking Rachel's advice, Natalie and Mary run for the dining car. They make it safely there, and welcome Isaac, Miria, and Czes when the three of them enter. Mary embraces Czes, happy that he is safe.
When the train finally arrives at Pennsylvania Station, Mary and Natalie, along with the rest of the passengers, are forced to stay at the station while the police conduct a two-hour long investigation. They meet with Manfred, and go to thank Rachel for her actions on the train. Mary informs Rachel that she wants to grow up to be a great person, just like her.
By February 1935, Mary has come to know Carzelio "Cazze" Runorata from the Runorata Family. While in New York she accompanies Cazze to the Martillo Family casino, where he plans on picking up Melvi Dormentaire. By chance, outside the casino, Mary recognizes Czeslaw, who is waiting with Annie outside the casino (due to hearing gunshots). She eagerly engages Czes in conversation, noting that she is taller than him (much to his embarrassment).Mary apologizes, and tells him how grateful she was back on the Flying Pussyfoot for his company; without the courage he gave her, she might have died. Mary then pulls Czeslaw into a hug. Standing behind her, Cazze asks if 'that boy' is the one who rescued her back on the train, and tells Mary to wait outside while he and his companions enter the casino to 'visit' Melvi and his hired-bodyguard Felix Walken.
Mary and Czeslaw's reunion is short when Melvi and Claire emerge from the casino. Czes, who is terrified of Claire, runs away "at full speed," leaving a mystified Mary in his wake. She notes that Annie has also disappeared, and feels that there is something 'off' about the situation. Carzelio, who has also reemerged, thanks Mary for waiting and tells her that he'll accompany her home. At Mary's thanks, he brushes it off, explaining that he has a letter from his grandfather he needs to deliver to her father as it is.
Czeslaw Meyer: A friend whom Mary met aboard the Flying Pussyfoot, and unbeknownst to her, an immortal. The two played with each other while on the train, running about with abandon. Mary views Czeslaw fondly, and is extremely grateful to Czeslaw for giving her courage on the train. The impression he made on her was strong enough that she still remembers him years after the incident.
Carzelio Runorata: Mary is acquainted with Carzelio, probably through her father's connection to the Runorata family, and like all of his friends she calls him Cazze. The two traveled together to pick up Melvi and Felix, but he went in without her when she reunited with Czeslaw. After picking up Melvi he took her home, which she thanked him for.
Natalie Beriam: Mary appears to have a close, loving relationship with her mother. Natalie is willing to risk her own life for her daughter, standing up to the hijackers in the dining car when they ask where Mary is, demonstrating the love she has for her daughter. Mary refers to her mother as 'mama'.
- It is likely that Mary was born in 1921, since in 1931 The Grand Punk Railroad: Local she is described as looking around the same age as Czeslaw, who was around ten years old when he drank the elixir.