Marilyn is one of the three Shadow Sirens in Paper Mario: The Thousand-Year Door. She and her sisters, Beldam and Vivian, work alongside the Secret Society of X-Nauts to find the Crystal Stars to open the Thousand-Year Door. Marilyn and her sisters appear several times through the adventure, making many attempts to slow down Mario and his partners.
Many years before the game, Marilyn and her sister, Beldam and Vivian, were servants of the Shadow Queen, who was an evil demon that sank the town of Rogueport. After being sealed away by the four heroes, the Shadow Sirens started planning on how to get the Crystal Stars to break the seal on the Thousand-Year Door.
While on their search, Marilyn and her sister eventually came across the organization known as the Secret Society of X-Nauts. They soon got to know the leader, Sir Grodus, and joined him and his minions as nothing but hired help. Knowing how desperate to rule the world Sir Grodus was, the Shadow Sirens informed Grodus that the Shadow Queen would obey his every command were he to release her. They also let him know of the seven Crystal Stars which would unlock the Thousand-Year Door. Additionally, they told him about the Magical Map inside a treasure chest that can only be opened by someone who is pure of hear. Due to their evil nature, neither Sir Grodus or the Shadow Sirens could open the chest, so they decided to get someone else, Princess Peach, to open it.
After the failed attempt to abduct Princess Peach and steal the map from her after the chest was open, they had no choice but to flee and return to the X-Naut Fortress. Later, Mario and his partners defeated Hooktail and obtained the Diamond Star, an X-Naut soldier informed Sir Grodus of this and he sent the Shadow Sirens to ambush Mario in the Boggly Woods to steal the map from him. Before ambushing him, Sir Grodus gave them a sketch of Mario so that they would know what he looks like. Beldam managed to lose the sketch and as a result Beldam and Vivian don't recognize Mario when they come across him. And due to the fact Marilyn cannot speak, Beldam cannot process what she is saying. Eventually managing to find the sketch, they recognize Mario as he is returning to get the necklace which belonged to Flurrie. The Shadow Sirens then introduce themselves, with this being the only time Marilyn speaks in the game, and state their purpose to steal that magical map. Afterwards, the Sirens engages Mario and his partners in a combat and are defeated. Beldam blames their defeat on Marilyn and Vivian telling them to prepare for a world class punishment. The Sirens flee and return to the X-Naut fortress, leaving Flurrie's necklace in their wake.
At the X-Naut Fortress, Sir Grodus ridicules the Shadow Sirens for their failure in retrieving the map. Beldam then tells him about the Superbombomb, which will bring Mario to a quick end. Although Sir Grodus is displeased with the previous failures, he trusts Beldam's word and sends the Sirens to Twilight Town. After taking a well earned siesta, they wake to find that Vivian is no longer with them. Later, Doopliss comes rushing by after being defeated by Mario and Vivian, and is recruited to be a member of the Shadow Sirens.
Not seen again till Chapter 6 at Poshley Heights. Along with Beldam and Doopliss, seen at Poshley Sanctum stealing what they thought was Garnet Star. After stealing the star, the Sirens fled from the scene, with Mario and his partners about the chase them, but Pennington, the manager of the Sanctum, told them that the Star they stole was fake and the real one was hidden somewhere within the Sanctum.
By the time Mario and his partners reach the Palace of Shadow, they are confronted by the Shadow Sirens again. Beldom reveals they were tricked into opening the Thousand-Year Door by Doopliss, who was disguised as Professor Frankly. It turns out they managed to abduct the professor sometime during the events of Chapter 7 and stored him in a closet. After the Shadow Sirens engage Mario and his partners in another combat, they end up defeated again.
Later, after Mario and his partners defeat Bowser and Kammy Koopa after Sir Grodus' defeat, Grodus regains consciousness and take Princess Peach down to the Shadow Queen's resting place. Sir Grodus then tells Mario and his partners the true motive behind kidnapping Princess Peach, declaring himself the ruler of the world and releasing the Shadow Queen. Grodus presents Peach to the Shadow Queen to become a vessel for her spirit. After the Shadow Queen possesses Peach's body, Grodus orders her to destroy Mario and his partners, refusing and instead blasts Grodus to bits. After Grodus receives a second strike to the head, the Sirens appear and explain that they were the ones truly responsible for reviving her. The Shadow Queen then congratulates them for the work and offers Mario and his partners to come join here. They refuse and engage her in combat. Eventually managing to defeat her with the power of the Crystal Stars. After the Shadow Queen is defeated, Beldam, Marilyn, and Doopliss retreat.
Once the Shadow Queen is defeated, Beldam and Marilyn change their evil ways and decide to make good with their sister. Beldam apologizes to Vivian, promising never to treat her wrong again.
Unlike the other Shadow Sirens, Marilyn happens to be quiet and is rarely seen talking as being described by Goombella as the "strong, silent type". The only words she speaks throughout the game are "The Three...", otherwise only making "Guh" noises. Despite the fact she is not speaking in plain English, Beldam is able to understand her.
Although very dark and mysterious, Marilyn has still been shown to be loyal to her sister and will follow the orders given to her. It is later revealed that Marilyn is the favorite of the group, since Beldam will treat every other member of the group with as little respect as possible. It is not revealed if Marilyn is as sinister or cruel as her sister. Regardless, she still reformed at the end of the game.
Marilyn stands out as the strongest member of the Shadow Sires. She is physically strong and specializes in the power of lightning abilities. She is able to charge her attack power and if the player does not stop her she can increase her power, which may cause an instant kill. Like her sisters, Marilyn knows how to use the Veil ability, where she can sink into shadows. This ability is either used to hide or teleport.
- "I guess that's the middle sister. She seems like the strong, silent type to me. Hmm... I can't get a good read on this girl at all..."
- "That's Marilyn. I still can't really get a good read on her, unfortunately. She doesn't seem like such a bad type, if you ask me. ...But who knows for sure?"