Mephisto is considered to be Marvel’s version of the devil who rules over the pocket dimension of Hell. The Marvel devil had a hand in making Wanda’s twin sons, Billy and Tommy, after Wanda manifested their existence using her magic and unknowingly used two pieces of Mephisto’s soul to bring them to life.
In Episode 3, Jennifer Kale tells Teen that Agatha’s long lost son could be an agent of Mephisto, so there’s a chance the devil will be the twist villain in Agatha All Along. At this time, no one knows anything about Agatha’s mysterious son Nicholas Scratch, except the fact he’s not around anymore. As Teen is currently under the influence of a spell that keeps him from unveiling his true identity to both the audience and the in-universe cast, there’s a chance he may be Nicholas in disguise.
When Jennifer mentions the fact that Agatha’s son could be working with Mephisto in Agatha All Along Episode 3, Teen looks a bit taken a back. This could be seen as him being shocked that Agatha’s child would choose to work with the devil, or as an admission of guilt, because he’s really Nicholas and currently has a deal with Mephisto.