Elektra

Elektra Natchios was a Greek ambassador's daughter studying at Columbia, in love with a blind law student named Matt Murdock, until terrorists killed her father in front of her. She walked away from everything, trained with the Chaste under Stick, was rejected for the rage she could not surrender, and completed her education with the Hand instead. That is the whole tragedy of the character in one sentence: she got the discipline from the good guys and the contracts from the bad ones.

Frank Miller created her as the ghost in Daredevil's past and then killed her off, brutally, in eighteen issues. She refused to stay dead. Elektra has been resurrected, purified, corrupted, replaced by a Skrull queen, recruited by S.H.I.E.L.D., and eventually handed the Daredevil identity outright while Matt was in prison. Nobody in Marvel's street-level tier has a more volatile relationship with their own morality.

Frank Miller introduced Elektra in Daredevil #168 during the run that redefined the character, and then had Bullseye kill her with her own sai in Daredevil #181, one of the most famous single pages of the 1980s. Miller extracted a promise from Marvel that nobody else would revive her, which held for a while.

Elektra: Assassin in 1986, with Bill Sienkiewicz on art, is her artistic peak: a fractured, hallucinatory Epic Comics series that looks like nothing else Marvel has published. Elektra Lives Again in 1990 handled her resurrection on Miller's own terms.

Post-Marvel Knights she became a permanent fixture: a Skrull impostor took her place in the lead-up to Secret Invasion, she led the Hand during Shadowland's fallout, ran with the Thunderbolts, and took the Daredevil name and costume in Chip Zdarsky's run while Matt Murdock was incarcerated.