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NOTE: This article is about the incarnation of Scarlet Witch from the Marvel Cinematic film series. The mainstream version can be found here: Scarlet Witch.
Sometimes it takes a while, but sooner or later every man shows himself.
~ The Scarlet Witch speaking with Ultron.
Wanda Maximoff, known also by her alias Scarlet Witch, is a major character in the Marvel Cinematic Universe. She is first introduced as a minor character in Captain America: The Winter Soldier, before becoming a major antagonist-turned-supporting protagonist (alongside Quicksilver) in Avengers: Age of Ultron.
She is an anti-villain, merely driven by revenge against Stark Industries for tearing her family apart as a child, but eventually becomes a hero and joins the Avengers after realizing they are preventing Ultron's destruction of humanity.
Wanda Maximoff and her brother Pietro are, at first, located in a HYDRA base that the Avengers are taking out. Together, they decide to engage them in combat, and Wanda, realizing that Tony Stark (Iron Man) is here to take Loki's scepter, uses her mind powers on him, making him realize that his actions will eventually doom the entire team of Avengers, and that the images of his allies dying are the future of his legacy. Wanda smiles mischievously as Stark rips the scepter free, now feeling the need to complete his Ultron project to better defend humanity. She and Pietro escape the HYDRA base.
After Ultron is activated, he escapes and begins upgrading his body into a powerful new threat, also finding out about the twins. He calls Wanda and her brother forth, and she states that she is unable to read him. Ultron reveals his new body and invites them to join him in creating the evolution of humanity by destroying the Avengers. Ultron reads into their backstory, and Pietro and Wanda explain that they lost their family when they were children to a missile attack in which Stark Industries weapons were used, revealing why they want revenge on Tony. Ultron states that Wanda has the ability to tear the Avengers apart from the inside with her mind abilities.
When the Avengers locate the three, Wanda battles and uses her mind powers on Black Widow, Captain America, and Thor, but before she can do the same to Hawkeye, he stuns her with an electric arrow to the forehead. Quicksilver rushes in, grabs her, and escapes. After she recovers, Wanda says she wants to "finish the job" and invades the mind of Bruce Banner, causing him to transform into the Hulk and rampage through the city.
When Ultron begins uploading his mind through a tube into a newer, more organic body that he wants to use, Wanda realizes that she can read him - and is horrified to discover that Ultron believes genocide and destruction is the only way to evolve. Upon Ultron next attacking Captain America before the uploading of his brain into the new body is complete, Wanda and Pietro turn on him, realizing that he was wrong, and side with the Avengers. Ultron asks Wanda not to do this, but Wanda replies, "What choice do we have?"
After Ultron leaves and the Avengers have secured the organic body, Wanda explains that she doesn't like the idea of getting the body anywhere near Stark, as she still feels that Stark is to blame for the situation, that Ultron cannot tell the difference between protecting and destroying the world, and that he gets it from Stark.
Captain America leads Wanda and Pietro against Tony and Bruce's plan to upload J.A.R.V.I.S. into the body. Bruce grabs Wanda in a chokehold, but she blasts him out of the way in self-defense. Thor steps in and completes the upload with his hammer, thus creating the Vision out of J.A.R.V.I.S. and the body. The new creation is able to convince the reluctant heroes that he is on their side. With the help of the Vision, they set out to find Ultron, Wanda gaining a new and curious interest in the Vision.
It turns out Ultron's plan is to propel the city into the sky and crash it like a meteor into Earth, causing mass extinction and resetting life once again. As the city rises and the Ultron Sentinels take over the city, Wanda undergoes a breakdown and shamefully tells Hawkeye the whole thing is her fault, and that if she hadn't invaded Stark's mind, he would never feel the need to complete Ultron. Hawkeye says it doesn't matter anymore, and what matters now is the present and future, saying that if Wanda steps outside to fight, she will be an Avenger. After a minute, she becomes determined to rectify her mistake and joins the fight, using her telekinetic might to destroy the Sentinels. Joining the heroes to defend the mechanical core that keeps the city in flight so that every last living being can leave the island via SHIELD helicarriers, Wanda fastly destroys Ultron's army alongside her allies. One by one, the heroes split up to accomplish different tasks. Wanda tells Pietro that she will defend the core alone, and he leaves to help escort people onto the helicarriers while she continues to take on the drones single-handedly.
Ultimately, Pietro is shot down by Ultron, who had previously hijacked a Quinjet, causing Wanda to feel that her brother has been killed and break down screaming, her rage and grief destroying the remaining Sentinels in a powerful burst. After Hulk punches Ultron out and hijacks the Quinjet, Wanda confronts Ultron personally. He tells her that she'll die if she stays here, and she responds by saying she just did, meaning that she died on the inside, thanks to what Ultron did to Pietro, causing her to feel his pain. With that, she rips out Ultron's core with her hex ability, destroying his main body, saying that the pain she just gave him was how it felt for her upon her brother's demise.
Wanda isn't afraid to die, but the Vision flies in and saves her as the island explodes with everyone successfully off of it. After, Captain America, along with Black Widow, starts a training initiative for Wanda, James Rhodes (War Machine), Sam Wilson (Falcon), and the Vision, hoping to create the best defense possible for the Earth, and Wanda begins her training as a new Avengers.
Powers and Abilities
Probability Field Manipulation
Telekineses: Wanda has the ability to throw objects and enemies, as well as tear apart non-organic enemies such as the Ultron Sentinels. When she loses control of her emotions, her powers become greatly amplified and even more dangerous. This is demonstrated when she destroys dozens of Sentinels in one blast of rage and grief after her brother's death. In Civil War, she overpowers Vision and sends him through the ground, drops several cars on Iron Man, and holds up a massive falling Airport Control Tower to let Steve and Bucky escape.
Mind Manipulation: She is able to probe the minds of opponents and force them to experience hallucinations or turn them against each other, seen when she manipulated the Hulk into attacking a city.
Energy Shields Generation: She can conjure shields of energy.
She is played by Elizabeth Olsen who already starred in Godzilla (2014) alongside with Aaron Taylor-Johnson as wife and husband, while in the MCU they are twin siblings.
She is based on Scarlet Witch of the Ultimate Marvel Universe rather than the mainstream one.
Wanda is younger than Pietro by 12 minutes and is, in fact, the youngest Avenger by a significant margin (she is implied to be around college student age and is most likely only in her early twenties).
It was implied that Wanda has feelings for Vision, as she normally stares at him every time he's around, and she looks at him in amazement when he rescues her. It's confirmed by the writers that this is a nod to her relationship with him in the original comics, possibly hinting at it happening in future films. This is further expanded in Civil War, where they have developed a strong interest in each other.
It is strongly implied that Ultron had feelings for her, as he was completely aware that she was going to destroy him, yet was still concerned that she would die if she stayed here. This is because he considered her the ideal "evolved human", which is what Ultron had been striving to create in the first place.
Just like Pietro, her alias "Scarlet Witch" was never mentioned in the film, however she is called a "witch" by Tony Stark.
Elizabeth Olsen and Joe Russo have stated that Scarlet Witch is the most powerful of the Avengers, bar none, and has only barely scratched the surface of her potential. Even Paul Bettany (Vision's actor) admits that she is probably the only one who can defeat Vision. This is further demonstrated in the Infinity War trailer, which shows her taking on a Black Order member (revealed to be Proxima Midnight and presumably protecting Vision as a reversal of their roles from previous films.
It was then implied that Doctor Strange will take an interest in her and begin to mentor her in mastering the magic arts, although in the final movie it never happened.