Quicksilver

Pietro Maximoff, known also under the alias Quicksilver (1989-2015), is one of the two titular tritagonists (alongside Scarlet Witch) of Avengers: Age of Ultron, and a posthumous character in the WandaVision series. 
He is the twin brother of Scarlet Witch, and was experimented by HYDRA under the leadership of Wolfgang von Strucker. The experiments with the mind stone gave Pietro the ability to move at superhuman speeds, as his sister gain the ability to control and manipulate psionics. After HYDRA was defeated by the Avengers, Pietro and Wanda joined Ultron to defeat the superhero team until the twins defected upon learning of Ultron's true intentions. During the Battle of Sokovia, Pietro sacrificed himself to save his teammate Hawkeye, and his death was avenged by the heroes by stopping Ultron's plans.
Hawkeye honored his sacrifice by naming his infant child Nathaniel Pietro Barton, and Wanda continued to serve the Avengers until their civil war. During the Westview incident, Pietro was impersonated by Ralph Bohner while enthralled by Agatha Harkness, who made Ralph get close to Wanda to learn how she created the hex.
In Avengers: Age of Ultron, he was portrayed by Aaron Taylor-Johnson. Gabriel Gurevich played the character younger in the Disney+ tv series, WandaVision.