The Skrulls have a long and complex history in the Marvel Comics Universe. They are known for their conflicts with other alien races, most notably with the Kree, another powerful species.
In Captain Marvel, the Skrulls are depicted as shapeshifting extraterrestrial beings. Initially portrayed as antagonists, they are engaged in a conflict with the Kree but are later revealed to be refugees seeking a new home.
The comic book run, Secret Invasion, is well-known for being a crossover event where the Skrulls infiltrate Earth by replacing key superheroes. Now a Disney+ series of the same name is creating its own story within the MCU.
Every MCU Hero To Be Revealed as a Skrull
Phil Coulson (Captain Marvel)
No, this has nothing to do with Agents of SHIELD. During a flashback scene in Captain Marvel, a younger version of agent Phill Coulson was impersonated by a Skrull.
One of the Skrulls from the crashed ship in 1995 impersonated Coulson to infiltrate SHIELD and capture Vers. Nick Fury exposed the Skrull's deception and deliberately caused a fatal collision.
Att-Lass (Captain Marvel)
In Captain Marvel, Algenis Perez Soto portrays Att-Lass, a skilled member of the Kree Starforce, an elite military unit.
During a mission on the planet Torfa, a Skrull disguised himself as Att-Lass to prevent a Starforce member, Minn-Erva, from shooting other Skrull refugees posing as Torfans, but their true identity was eventually revealed, leading to a fatal confrontation.
Nick Fury - Talos (Spider-Man: Far From Home) / Gravik (Secret Invasion)
Samuel L. Jackson's presence was felt throughout Tom Holland's second Spider-Man film. The entire time audiences were meant to believe that it was the true Nick Fury portrayed by Jackson.
In the post-credits scene, however, it is revealed that Talos had been acting as Fury for (seemingly) the duration of the movie. Fury wasn't even on Earth, he was on a SABER space station in orbit.
Maria Hill - Soren (Spider-Man: Far From Home)
Alongside Jackson was Colbie Smulder's Maria Hill. The duo was back together for the first time since Captain America: The Winter Soldier, and even then Smulders' role wasn't as large since 2012's The Avengers.
Turns out, it wasn't Agent Hill and the reality was the Skrull husband-wife of Talos and Soren were helping Peter Parker out in Spider-Man: Far From Home.
Everett Ross (Secret Invasion)
To the surprise of most Secret Invasion viewers, Agent Everett Ross was quickly revealed to be a Skrull after being chased by Talos.
Actor Martin Freeman doesn't believe Ross has always been a Skrull this whole time though:
"I don’t wanna believe he’s been a Skrull this whole time, so I don’t."
Priscilla Fury - Varra (Secret Invasion)
Priscilla Fury's introduction was uniquely connected to the Skrulls. Varra first appeared, then shifted into Priscilla when Nick Fury arrived home.
Audiences were left wondering where Nick knew his wife was a Skrull but soon found out that he's always known.
Rhodey - Raava (Secret Invasion)
The Skrull reveal that may be the most comparable to some of the surprises in the Secret Invasion comics was James Rhodes (Don Cheadle) as a Skrull.
Speculation began early in the series, especially when Rhodey fired Nick Fury, that he wasn't himself. It was confirmed in Episode 4 that Skrull Raava is currently acting as Colonel Rhodes, fooling most of the US Government, including its President.
It hasn't been explicitly said how long Rhodey has been a Skrull, but there's a possibility that it has been for longer than fans may have initially thought.