Pietro Lensherr, a.k.a. Quicksilver, is the son of mutant supremacist Magneto and twin brother of Wanda. Magneto abandoned their human mother when the twins were children, taking them with him as he founded the Brotherhood of Mutants with Charles Xavier, whom the children regarded as an uncle. Pietro loved his father, but nothing he did was ever good enough for Magneto; perhaps because Pietro was a constant reminder that Magneto had once loved a human. Pietro continued trying to win his father's respect, becoming his lieutenant and participating in the Brotherhood's anti-human terrorism. His father in turn deliberately belittled him. With doubts placed in Quicksilver's mind by the X-Man Cyclops, Quicksilver helped the X-Men defeat his father when Magneto tried to trigger a nuclear holocaust.

With Magneto presumed dead, Quicksilver assumed leadership of the Brotherhood. Feeling guilty about his role in his father's fate, Quicksilver tried to recover Magneto's corpse from the military, but found only his helmet. The X-Men offered the twins membership, but they declined. Still, relations between X-Men and Brotherhood thawed, and when Weapon X captured the X-Men, the Brotherhood rescued them. During this mission, Quicksilver encountered S.H.I.E.L.D. director Nick Fury, whose decision to allow the Brotherhood to leave peacefully may have earned Quicksilver's trust, laying the foundations for their future dealings. Pietro was soon convinced by "Uncle Charles" to shift the Brotherhood's activities from bombings to less violent protests, targeting financial institutions and research facilities experimenting on mutant animals. Many of the Brotherhood quit, feeling his leadership was weak, so Quicksilver led them in stripping Pakistan and India of their nuclear arsenals to prove his strength. He and his sister also brokered a deal with S.H.I.E.L.D. to go on selected black ops missions in return for the release of mutant "political prisoners"In this capacity, they helped repel an invasion by the alien Chitauri.

When Magneto returned, his children feared his wrath and defected to the Ultimates. Magneto breached S.H.I.E.L.D.'s Triskelion base and shot Quicksilver in the kneecaps as punishment for his betrayals. Quicksilver swiftly recovered from these injuries. Always close to each other and raised among mutants, now that they were surrounded by humans the twins grew closer, affecting an air of detached superiority towards their new teammates. In spite of this attitude, they proved crucial members of the team. Quicksilver played a key role in the capture of rogue teammate Thor. When Emma Frost's mutant students attempted to break into the Triskelion, Quicksilver battled fellow speedster Northstar, finally meeting someone as fast as himself. Quicksilver recently helped the Ultimates disarm a Middle-Eastern nation.