Kathryn Hahn's Agnes has been the talk of the fandom since her appearance in the trailer, and it hasn't died down yet! She's been playing the role of nosy neighbour as Wanda and Vision travel through eras, but several clues hint at her real identity: Agatha Harkness, witch extraordinaire. In the Marvel comics, Agatha is a witch who survives the Salem witch trials and mentors Wanda in using her magical powers. Although Agatha is best known as an ally to Wanda and the Avengers as a whole, she eventually meets her devastating end when she's killed by Wanda after the younger witch loses some of her memories and sanity.
In WandaVision, Agnes quickly inserted herself by Wanda's side, setting herself as a close friend and helper like her role in the comics. Much like her character, she sports a noticeable brooch and seems to know more than she lets on. The most damning hint to Agnes's real identity comes in a preview of the upcoming Halloween episode. As we saw in the previews, everyone seems to dress as who they truly are. Wanda wears her classic Scarlet Witch costume, Vision forgoes his human glamor, and Agnes sports a witch costume.
If Agnes does reveal herself to be Agatha Harkness, the real question is will she be friend or foe?