Sharon Carter

Sharon Carter is S.H.I.E.L.D.'s Agent 13, one of its best field operatives, and the woman who has spent sixty years of publication being both Captain America's partner and the person cleaning up after the institutions he trusts. Originally written as Peggy Carter's younger sister and later retconned to her niece, she has been declared dead, held prisoner in another dimension, mind-controlled into shooting the man she loved, and abandoned by her own agency more than once.

That last part is the modern take. Marvel and the MCU have both leaned into Sharon as the agent who gave everything and got nothing: her screen version spends five years as a fugitive arms dealer in Madripoor after helping Steve Rogers, and returns as the Power Broker with a legitimate grievance and an illegitimate business model.

Stan Lee and Jack Kirby introduced Sharon in Tales of Suspense #75 in 1966 as Agent 13, and she quickly became Captain America's most significant long-term partner. Marvel wrote her out with an apparent death in the 1970s, then brought her back in the 1990s with a bitter, hardened characterisation shaped by years presumed dead.

Ed Brubaker's run gave her the worst assignment in her history: under Doctor Faustus' mind control, Sharon fired the shots that killed Steve Rogers in Captain America #25. Secret Empire later trapped her in the Darkforce dimension and aged her decades, and she has since been restored and reinstated. The retcon of her relationship to Peggy, from sister to niece, was made to keep the timeline workable.