Quicksilver
Pietro Maximoff, otherwise known by his codename of Quicksilver, was born in Sokovia along with his twin sister Wanda as the orphaned children of a former cleaning woman turned freelance mercenary agent named Magda Eisenhardt. When they were ten, their adoptive parents, Django and Marya Maximoff, who were old friends of Magda before her passing, died themselves as a result of a bombing caused by several Stark Enterprise missiles stolen by HYDRA terrorists, and trapping the twins beneath a pile of rubble for several days until their rescue. The attack from then on caused them to foster a burning hatred toward Tony Stark due to their perceived belief of him being responsible for building the missiles in the first place. Years later, Pietro and Wanda volunteered to undergo experiments involving the Chitauri Scepter, little realizing that they were now under the power of the same entity which caused their traumatic bombing in the first place, HYDRA. The scepter gave the twins newfound abilities which they later used in their vendetta against the Avengers and Baron Strucker. However, after learning that their second ally, Ultron, was planning to wipe out humanity, they escaped his subjugation and joined the Avengers to stop his diabolical plan once and for all. However, Pietro could not escape Ultron's vindictive wrath, and was gunned down by the mad android while trying to save the life of his new teammate Hawkeye and a young Sokovian boy.
Some time later, following the fallout of the Civil War engineered by Baron Zemo, it was brought up to light that Pietro and Wanda had actually been sired by the mutant overlord Magneto during his union with Magda, and that all the Chitauri Scepter's energies had done was simply activate their latent mutant powers. Wanda would take the initiative to honor her brother's memory and demand answers from Erik about their past scars and subsequent abandonment from their father. She would then go on to merge her Mutant Gene with that of the Inhuman Gene, giving her even greater abilities and allowing her to bring back Pietro from the dead to protect her from further danger during the Infinity Wars.