Jacques Duquesne, The Swordsman was just another second-tier baddie when I first started reading Marvel comics. He was a henchman with personality, almost always teaming up to take on the Avengers or some individual member such as Captain America. He became an Avenger briefly only to betray the team and then he became associated with the Mandarin and Power Man. Later it was revealed he'd had something significant to do with Hawkeye's history and suddenly the Swordsman became a much richer and inviting character, a baddie with backstory.
Marvel saw something in the guy though, as he made a momentary appearance in the hundredth issue of the Assemblers comic which brought all the members back together. Later he returned alongside Mantis and made a pitch to redeem himself. He did that, but soon enough died during the sprawling Kang War.
He made quite an impression on me though in his short heroic career, and the dopplegangers Marvel has trotted out since Duquesne's demise are the Swordsman in name only alas. In a universe that prides itself on tragic heroes, Swordsman was one of the most tragic.
Below is a gallery of most of Swordy's cover appearances. He's featured on very few of them. I've also included at the end a glimpse of two of his replacements, though how they fit into the fabric of Marvel is lost on me these days.