The Final WandaVision Cheatsheet – E! On-line
Why Wanda just became the Scarlet Witch
Jimmy Woo confirmed on WandaVision that Wanda still hasn’t had a funny superhero nickname. Viewers know her as the Scarlet Witch, but until the end of episode eight, she was never called that on-screen after Agatha got the full picture of her powers.
Usually portrayed as a mutant, like the X-Men, she started out as a supervillain alongside her brother Quicksilver, but then switched sides to join the Avengers. She once had magical twin sons, and those sons turned out to be part of the demon, courtesy of the demon super-villain Mephisto. In later comics, she is no longer a mutant, but was born a witch.
In the MCU, the Scarlet Witch seems to be more of a concept than a specific person, based at least on how Agatha bestowed that title on Wanda. Wanda had visions of a witch when she first touched the Mind Stone during the HYDRA experiments, and Agatha named her after learning how Wanda created her sitcom world and made vision entirely out of magic.