Within the show, WandaVision, Agnes shows up early on as a nosy neighbor who threatens the reveal of Wanda and Vision's true natures, but is otherwise a quirky ally. Episodes 7 and 8 reveal her true nature, namely that
She's Agatha Harkness, a witch from the Salem, who has been manipulating Wanda in an effort to learn where she got the power for such a massive transformation, and it was Agatha all along.
That's reasonable enough, except that what we're shown seems to indicate that Agnes was there before the transformation of the town. I haven't had a chance to rewatch the episodes for further clues (and the new baby means I kept missing bits of what was going on), and what I've found online hasn't helped elucidate, but was Agnes in the neighborhood prior to things transforming? If so, was it because she knew something was going to happen, or just that she was in the right place at the right time?