I think much of westview is controlled by Wanda's subconsciousness. It is almost like a dream playing out in real life...it all comes from her but she is not consciously deciding what people say or do (like you dont have control of that in a dream). When she feels threatened, her subconscious will change something or when the outside world threatens her, she directly decides what to do.
I think Pietro is the Xmen version that Wanda's subconscious plucked out of his universe. His role in Westview is to rationalize Wanda's behavior...which he does later in episode 6. I think this because he shows up right after Vision confronts Wanda about the morality of what she is doing, forcing her subconscious to defend itself by taking extreme measures. Her actual brother is dead, so it needed to find someone with similar abilities and control that person to what Wanda needs...the person she trusts most in life to tell her what she is doing isn't wrong.
So basically, Pietro is the xmen version being controlled by Wanda's subconscious like everyone else in westview to serve a purpose. He even says that most people keep their same personality...and his personality is very much like his character in the Xmen movies (a man child, likes pranks, chugs soda's, etc).
