When it comes to who is behind all the insanity of WandaVision, the main suspect is Mephisto. The demon, who is known to go by Lucifer and the Devil, has a long history of doing wicked deeds in Marvel history and is the biggest bad (aside from Magento) in Wanda's past. His role in the death of Wanda's children is the main reason why he's suspected to be the one pulling her strings in WandaVision, and the numerous clues to his presence seem to support the idea. One big hint: the colour red. Devil red is a prevalent pop of colour throughout the series, and while you could argue it's to represent Scarlet Witch and Vision, it seems more like it's Mephisto letting everyone know he's in every scene. If it is the big red demon behind everything, the show would definitely be setting up his role as Marvel's next big bad.