Karl Fiers, or the Finisher, as he was better known to his employer and his employer's enemies, was an assassin employed by the fascist/ Communist Red Skull double, Albert Malik. So efficient was he at his job, the Finisher only needed a cloth from his intended victim to track them using his skills.
Upon learning Richard and Mary Parker were CIA spies in his criminal organization, Malik ordered the Finisher to take care of them. It was he who sabotaged their plane with a timed bomb before they went on a mission for the Red Skull (Malik), thus effectively resulting in their deaths.
The Finisher was created by Stan Lee and Larry Lieber, with his first (and only physical appearance as of recorded ) being The Amazing Spider-Man Annual #5 - At Long Last...The Parents of Peter Parker! in 1968.
Major Story Arcs
The Quest for Innocence
Years later, Malik would call upon the Finisher again; this time to handle the costumed interloper Spider-Man, who was searching for proof to clear his parents' names (Richard and Mary Parker), whom were branded as traitors to the U.S. due to their involvement with the Red Skull (the information that they were double agents being obviously kept out of public knowledge)
After Malik gave him a piece of torn cloth from Spider-Man's costume, the Finisher set out to take care of his boss' foe once and for all. While in his specially equipped limo and tracking his intended prey, the Finisher launched a missile that locked on to Spider-Man, thanks to a device in the Finisher's limo and the cloth sample that specifically tracked the same type of cloth. Spider-Man leapt away from the missile's path, and led it straight back to the Finisher's limo, thus effectively killing him. Before he could die, Spider-Man managed to get a confession out of the Finisher, who detailed his role in the death of Spider-Man's parents, as well as stating that his parents were innocent of all charges.
Spider-Man subsequently found the Red Skull (Malik), and after a second battle, managed to find the physical evidence needed to clear his parents' names.
Novels & Other Media
In the Sinister Six series of novels written by Adam Troy-Castro, it's revealed Karl had a brother Gustav, who called himself the Gentleman, a mastermind criminal that actively hates the majority of humanity for lacking what he calls "usefulness" to society in general. Gustav would also be a brutal killer and planner just like his brother if not more so, planning a time bomb to kill the wife of a detective that was hounding him, or almost killing the man who would be known as Wolverine while they worked in the same organisation. It was the death of his brother, the Finisher, that prompted him to seek revenge upon Peter Parker, Spider-Man. Gustav would set up major relationships with the Sinister Six, using them to attack the city in terror attacks to lure out the webslinger and attempted to wipe out all computer records of Wall Street financial records, as well as spreading a special chemical that would have destroyed all ink it would have came across. All of this was designed to destroy all major sources of money and cause a massive crash that would have allowed the Six to profit massively from the chaos. He was stopped and killed by the Chameleon in disguise, and his final deadly surprise: a bomb hidden in the Parker home: was disarmed by Wolverine, whom had caught wind of his scheme but chosen to keep them safe rather than try to get his revenge.
It's unknown if these novels are indeed canon to the comics or not: Gustav would later appear within the pages of Civil War II but his past or death wasn't not mentioned. The Gentleman also appeared in The Amazing Spider-Man series, starring as end credit cameos. His role apparently would have followed the comics closely by having him mastermind the Sinister Six coming together but as the series was cancelled, this isn't confirmed.
Powers & Abilities
Finisher had no powers, but he had a innate sense of timing and planning, allowing him to set up almost to the letter traps and bombs. Those who crossed him or the people who employed him usually were taken out before they could even know it, and he preferred to stay behind the scenes when killing, not getting his hands dirty. He was also skilled at building advanced technology, building a rocket and a tracking system that managed to find Spider-Man simply through a piece of his costume.
Ultimately, his weakness was that he was too reliant on his technology to get the job done: if it failed he had no back up, and Spider-Man was able to quickly outclass him when he was able to use that to his advantage.