Sinead Starling is a member of the Cahill family, belonging to the Ekaterina and Madrigal branches. She is one of the Starling triplets, who formed a team during the Clue Hunt. Sinead is also a former Vesper, and held the title of Vesper Three during the events of Cahills vs. Vespers.
As mentioned in the books, she wears preppy clothes, like khaki pants, button-down shirts, and loafers. She usually wears her long, auburn hair in a ponytail and has freckles. According to Dan Cahill, the triplets look like "part of a cloned Ivy League lacrosse team." Also, in The Medusa Plot, Dan says that she is "tall, strikingly pretty, and brilliant." In A King's Ransom, it was mentioned that she had green eyes. In The Dead of Night, Jake Rosenbloom insulted Sinead, Ian, and Evan by calling them the beauty contest winner (Sinead), the posh boy (Ian), and the geek (Evan).
In The Maze of Bones, the Starlings followed Amy and Dan Cahill to the Franklin Institute. Once Amy and Dan found the Clue, Sinead took a picture of it on her camera phone. Amy and Dan suspected that The Man in Black had set a bomb at the main entrance. Despite their warnings, the Starlings left through the main entrance. A sonic detonator blew up, and buried the Starlings under rubble. Since then, Ted could only see light and dark, Ned had huge headaches, and Sinead was left with scar tissue all over her body. They were hospitalized, and it took months for them to recover. Finally, when they got out of the hospital, they were grounded by their parents, causing them to be eliminated from the contest. Or so it is believed by the majority of the Clue hunters throughout most of the series...
In Book 10, the Starlings return - their first appearance is at the Globe, where they are dressed as ninjas and are sneaking around during a Romeo and Juliet play that Amy, Dan, and Nellie, the Holts, the Kabras, Jonah Wizard, and Alistair Oh are also attending. It is shown that Sinead cares very much for her brothers' injuries from the Franklin Institute, she helped her brothers up until now. She makes an alliance with Alistair after they told him they had stolen Bae Oh's Clues (at first Alistair was not interested). They discovered the Shakespeare Clue and captured Dan but were stopped by Hamilton Holt. In the final part it is revealed that she only wants the serum to treat her brothers injuries and the only Clue she and her brothers knew was Zinc, which they stole from Bae Oh.
In this book, she is living in Amy and Dan's guest house. She has also grown very close to Amy after the Clue hunt. When Amy and Dan go to Italy, she stays behind at the Cahill Command Center with Ian and Evan. She also gives them a smoke bomb because she thinks that it might be useful against the Vespers (they later use it against an art thief).
Sinead stays in the Command Center and arranges the fake passports for Amy and Dan, and identifies that the lizard comes from Argentina. She and Evan have a fight about Evan putting the lizard on the Message Board.
Sinead is still working in the Cahill Command Center. She accesses the data deleted by Erasmus. She and Evan help Amy and Dan and Sinead tells them her suspicions about Ian as Vesper Three and the death of Erasmus and Luna Amato. She also emails a message to Luna, alerting her to start her ruse.
Evan finds out that she altered a photo of the hostages and changed the characteristics of the lizard in order to frame Ian, and that she is Vesper Three. She is also the narrator in Chapter Two, where she is packing and getting ready to leave. She tries to block the signal in the car Amy, Dan, Jake, Atticus and she are driving in to prevent them from finding out that she is the mole, but she fails and Amy has enough time to read Evan's message outside the car. When Amy confronts her she says it's for her brother, she wavers for a minute when she is reminded that her brother is a hostage. But then she says he won't be harmed and he will understand once she explains everything to him. When Sinead compares Amy and Dan's relationship to nothing, Amy kicks her and Sinead recovers and runs. When she is cornered she brings out her gun and threatens to shoot, but Amy doesn't believe her and almost gets shot, but gets a blister on her neck from the guns' close range. When they fight, Sinead starts tearing at Amy's hair and eyes and tries to strangle her, once she gets the upper hand. When Jake comes, Sinead runs and is not mentioned again in the book except that they are putting out a watch for her and Ted telling the hostages that Sinead liked Riley McGrath (Damien Vesper III).
Sinead is spying on Amy, Dan, Jake and Atticus when Isabel Kabra makes an AWW speech, disguised as a woman with a long hat. Sinead saves Amy's life by shooting one of Isabel Kabra's bodyguards and she rejoins the Cahills' team to fight the Vespers' organization and help destroy the Machina Fini Mundi. At the end of the book, she is seen talking to Ned and Ted and mentioned experimenting with treatments for them, and that she wouldn't use the serum, because she saw the effects that the serum brought to Isabel Kabra.
Sinead is being forced to unleash the virus.
Sinead does not have an official card but does appear on Card 303: Magnifying Glass.
Clues known to have
- Zinc - confirmed when Isabel is threatening her in Into the Gauntlet and Alistair steps in, saying that the Starlings only had this Clue, and that they stole it from Bae Oh, Alistair's uncle.
- She was formerly known as Vesper Three.
- Sinead was accepted into college at 14, but stayed in high school to play lacrosse.
- Sinead spent a summer training hackers at the Ekat stronghold in Cairo, Egypt.
- Sinead has attended four Taylor Swift concerts.
- The short story Rapid Fire 2: Ignition states she is 16 at the time of Grace's funeral.