Wanda Maximoff
Wanda Maximoff was first introduced in Avengers: Age of Ultron as a villain working alongside her twin brother, Quicksilver. However, after his death, she joins the Avengers and becomes a crucial team member. She is one of the most powerful characters in the MCU.
The name Scarlet Witch was not initially revealed in the MCU due to a rights issue with Fox, which owned the rights to the X-Men characters. However, in WandaVision, she is finally referred to as Scarlet Witch, marking a significant moment for this character.
