WandaVision episode 7 revealed Agatha Harkness as the villain behind Westview's biggest mysteries, but the twist failed to answer the question regarding Pietro's real identity. Showing up unannounced, subtly wrong in numerous ways, and with a different actor than the MCU version, Pietro has made callous or even hurtful comments, and seems to have sinister motivations of his own. In many ways, Pietro is both too close to, and too distant from, being real to be Agatha's creation.
If Pietro were simply pulled from Wanda's memories - if, indeed, that is something that Agatha is capable of doing - he would appear as Wanda knew him, and would behave in ways consistent with that characterization. And if Agatha were mind-controlling Pietro, and had chosen a stand-in from Westview for that purpose, she would have had to grant him the powers of the original Quicksilver, an impressive feat to say the least, and his characterization would have been a guess - the original Quicksilver was not a public figure since he died before the Sokovia Accords were introduced. Nevertheless, the SWORD observers immediately recognize who the substitute is supposed to be, while simultaneously having no 'real' identity for him on their cork board.
Since nothing in Westview is made from whole cloth, Pietro must be either an existing person made to appear to be Pietro, some manner of resurrected Pietro, or another version of Pietro from the multiverse. The first category is the broadest, and includes both Agatha creating an imposter from a normal human, and a shape shifter, whether supernatural (like Mephisto) or alien (like a Skrull). If the former, questions of Pietro's powers are waived - after all, a Avengers: Age of Ultron, the line about Quicksilver dying in the street "for no reason" has the hallmarks of disdain typical to that villain. Although Agatha is shown to be manipulating Pietro in some form, her villain reveal brought about more questions rather than answers.
Vision's return notwithstanding, discussion around bringing the dead back to life through magic isn't something that can happen under ordinary circumstances. Since Wanda was shocked by Pietro's return, this seems unlikely to be her doing. With the discussion of the multiverse that has shown up in WandaVision, as well as in promotional materials for Spider-Man 3, an extra-dimensional duplicate could answer a great deal of the questions about Pietro, including furnishing him with the backstory details and answers that he has, if Agatha has been controlling him.
With different memories of Sokovia and a chip on his shoulder that doesn't reflect the more mature Pietro from Age of Ultron, one thing's for certain - Quicksilver knows too much and too little. If his presence is the work of some cunning manipulator or the machinations of some diabolical presence, then it's the clumsiest manipulation yet in the show. If it's a stand-in, then whoever is trying to mimic Pietro didn't know him well, and the motives for doing so are hazy at best. With only two episodes to go, WandaVision will have to answer all of these questions and more in very little time.