Portrayed by Olivia Colman, the character is both friend and foe to Samuel L. Jackson's Nick Fury, who has returned to Earth after years spent in Space in order to face the threat of a clandestine take-over of the planet by shape-shifting aliens, the Skrulls.
The Academy Award winner spoke about her character at the Disney+ show's global press conference, and teased what fans can expect in the six-part limited series.
Who is Sonya Falsworth?
Sonya Falsworth is a new character to the MCU and she was created for the purpose of the espionage thriller, though she does have a connection to the franchise before her debut in Secret Invasion.
The character appears to be the relative of James Montgomery Falsworth, a British soldier who was part of the Howling Commandos that worked with Steve Rogers (Chris Evans) during the events of Captain America: The First Avenger. The link between the two is not confirmed, but it would be a mighty coincidence if it wasn't.
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Sonya is an MI6 agent with many friends in high places, Fury included, and she is not averse to using violence to get what she wants.
Olivia Colman shares details of Secret Invasion character
Colman was keen not to give too much away when it came to her character during the press conference, but she revealed that Sonya is an MI6 agent with a history with Fury.
"They have a lovely friendship. Although, she does quite like to bully [Fury] with her heavies," the Crown star explained.
Getting increasingly worried about giving away spoilers, she went on: "Doesn't strike me as much of a friendship, but they've got history. They trust each other, which is, I think, I mean, I'd say that... Maybe they don't. Oh, God."
Joking that she'd gone "very red" after getting so close to slipping up, Colman added that she "can't remember anything about it" as a way to avoid saying too much more.
Checking in with Marvel Studios president Kevin Feige, Colman later added: "Sonya works for MI6. That's okay, isn't it? And she likes wearing red. And she's quite funny. And she's potentially a little bit not that nice sometimes."
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Jackson described Sonya as "possibly the most dangerous woman in the UK", and viewers will get the chance to see exactly what he means when the show is released.
Why has Olivia Colman joined the MCU?
Colman has always wanted to be part of the Marvel franchise for so long that she'd regularly ask her agent about getting a role.
She said: "After every Marvel film that's landed in cinemas, I found my agent and said, 'Please can I be in a Marvel?' And finally, either she got fed up with me calling her or it happened.
"So it wasn't 'why Secret Invasion?' Secret Invasion called. Any mention of the word Marvel and I went, 'Yes, please.'
"So that's why, and I had an absolute ball, and it lived up to everything that I was hoping for."
Secret Invasion premieres on Disney+ on Wednesday, 21 June.
Watch: Samuel L. Jackson and Olivia Colman discuss Secret Invasion