Maddy De Costa

 is a character on Treason. She is portrayed by Oona Chaplin.

Biography
Maddy is an ex-army officer who is now an occupational therapist working with veterans.

She is Adam Lawrence's second wife. She is devoted to him and is the stepmother of his two kids, Callum and Ella. Maddy wants answers when Adam's promotion to M16 chief jeopardizes their family, especially after discovering her husband has been in contact with his ex-lover, Kara Yusova. She begins secretly monitoring Adam after her suspicions are roused.

While in the military, she served alongside Dede Alexander in Afghanistan. When the agency launches an investigation into Maddy's husband, Adam Lawrence, Dede takes on the case to protect Maddy and use her as an asset.

Throughout Treason
When Maddy's daughter, Ella is kidnapped, she worries about the safety of her family. She shares these concerns with her friend in the CIA, Dede Alexander. They previously served in the military together, and Dede had saved Maddy's life. Dede gave Maddy a pen that could record sounds within 10 metres, which she used to record Adam's phone call with Kara. Later, Maddy followed Adam when he met up with Kara, and she took a photo of the two. The recording and the photos were handed to Dede.

Maddy plants a gift card with a tracker in Adam's wallet to help the CIA track him. They follow him when he meets with Kara again. Maddy joins Dede in the van while they track Adam. She overhears Adam's conversation with Kara, where she tells him it was the CIA who kidnapped Adam. Maddy is angry at her friend for betraying her. The two argue outside the van, and are spotted by Adam. Maddy feels betrayed by Dede, as it was the CIA who kidnapped Ella and Dede had been using her to get information. However, Dede told Maddy that Ella was not her daughter by blood.

Adam takes his children to a house, where they hide. They are joined by Maddy. While Maddy and Adam argue, a shooter watches them from outside. Callum, who is in bed, hears noises and goes outside. He is grabbed by Kara, who drags him inside, much to the surprise of the others. She reveals that Martin ordered a hit on him. Kara and Maddy run outside to create a distraction. Kara runs into the shooter's car, dodging the shooter's bullets. Just as the shooter aims his gun at Kara, Maddy shoots him from behind. They move his body out of the way, and leave the house.

Adam, Maddy, Kara, Ella and Callum go to Kit Harper's house. Kit is a lawyer who was Adam's ex-wife's best friend. They stay at his house. Maddy has a hard time coping with the fact that she killed someone, while out of the military. Adam apologises for putting her in that position, and reassured her that she is on the right side. They are interrupted by Callum asking where his stuffed toy Samson is. It is revealed that Samson was found at the house near the shooter's body. A man calls Martin to inform him, and they believe Adam and Kara are working together. Martin calls Dede and arrange to meet up.

Adam talks to Kit about Martin's files containing evidence of a Russian plot to run Kirby, and that he suspects Adam of being a double agent named Dorian. Martin wants to let Kirby win and expose the plot to strengthen his position at MI6, but Adam wants to force it out in the open first, so that Martin and Kirby's careers end and Audrey can still be PM. Kara suggests offering to give Audrey intel on Kirby in exchange for immunity. They decide to have Adam and Kara search for Martin's files, and have Maddy make a deal with Audrey.

When Adam, Kara and Maddy go to retrieve the files with information about Dorian and the 5 men who were killed in Baku, there is a shootout. Dede holds Adam at gunpoint and calls out to Maddy, who is holding the files and hiding with Kara. Maddy runs out, and Dede shoots Adam. Maddy is distraught and wants to save him, but Kara tells her that he is dead. The two run away. Kara tells Maddy that the CIA would not shoot someone on UK soil unless they had authorisation, which means Martin will be looking for them.

Appearances

 * Treason

Trivia

 * Maddy says her last name is Spanish, and she speaks Spanish.