The speedster proved himself incredibly useful in the 2014 X-Men film by breaking Magneto (Michael Fassbender) out of a government prison, and then he returned to the X Mansion ten years later during the events of X-Men: Apocalypse, during which he decided to stick around and become a member of this adoptive family.
Now with another decade of experience under his belt, he has become more of a team player who wants to help the X-Men build a better public reputation.
So per Evan Peters’ remarks to Screen Rant, it seems that Peter Maximoff has toned down some of his most eccentric qualities since we first saw him during the events of X-Men: Days of Future Past.