While the Hex’s origins aren’t extremely clear just yet, Agnes arrived almost immediately in WandaVision’s show-within-a-show footage, giving viewers a quick introduction to her ever-curious and questioning nature.
(And there are even theories about Agnes and her purse at the talent show.)
Each moment where she’s popped up since then has been deliberate and meaningful for both viewers and the show’s central super-couple, save perhaps for her presence at the talent show.
From appearing soon after Elizabeth Olsen’s Wanda found the helicopter drone to showing up in Episode 5 soon after Vision pointed out her predictable patterns, Agnes has made it clear her appearances are all about timing, and it’s easy enough to argue that she’s motivated to guide Wanda and her children to darkness.