A twin from Sokovia who volunteered for an experiment and came out faster than anyone alive.
Pietro Maximoff is Wanda’s twin brother, played by Aaron Taylor-Johnson, with Gabriel Gurevich as the boy he was. The two of them are introduced as Hydra assets who volunteered to be experimented on with the Mind Stone, and came out of it changed: Wanda with her powers, Pietro with speed. They hate Tony Stark, whose bombs killed their parents, and that is why they start the film on Ultron’s side.
He is in the series for one film, and the reception was lukewarm, with the common complaint that neither he nor his power was developed enough, either against the comics or against the version Fox had put on screen a year earlier. This catalogue files that one separately, under his other name, Peter Maximoff.
Minor spoiler
In the final fight with Ultron he dies saving Clint Barton and a Sokovian boy, which is the film’s one real cost and the thing Wanda carries into everything after it.
Show detailsThe other Pietro
In WandaVision a Pietro turns up at Wanda’s door, played by Evan Peters, who had played Fox’s Quicksilver. For a few episodes it looked like the two film series had just joined hands. They had not: he is Ralph Bohner, a resident of Westview forced by Agatha Harkness to impersonate Pietro to get close to Wanda inside her invented town. Wikipedia counts the series among the character’s appearances on that basis, and so does this entry, with the actor’s note saying what he was actually doing.