The introduction of a new Parasite to Supergirl season 4 corrects the mistake the showrunners made in killing off the character during the show's second season. The classic Superman villain first appeared in the Supergirl episode "Changing" and died at the end of his premiere episode.
There have been several villains who have used the code-name Parasite in DC Comics history and all of them have shared the same disturbing power of absorbing the life force, memories and superpowers of anyone they touch. The first was a petty crook named Raymond Jensen, who was mutated after being exposed to toxic waste. The second was a janitor at STAR Labs named Rudy Jones, who was likewise changed after a lab accident. There was also a pair of twins - Alex and Alexander Allston - who were transformed into Parasite clones by the villain Ruin. Finally, in the New 52 reality, The Parasite was a bike courier named Joshua Allen, who changed after trying to attack an alien creature with a live electrical wire.
The Arrowverse version of Parasite introduced in "Changing" had the same powers and general appearance of his comic book counterpart. His origins, however, were drastically changed from the original comics. In fact, this version of The Parasite owed more to Venom and the movie The Thing than he did to the character from Superman.
The first Parasite on Supergirl was Dr. Rudy Jones - a climatologist working at a remote research facility in Norway. It was here that Dr. Jones became infected by an alien worm, who had been hibernating within the frozen body of its previous host, a wolf. The worm empowered Dr. Jones, giving him Parasite's famous energy draining touch. It also made him start referring to himself in the third person, much like Eddie Brock's symbiote pal.
Dr. Jones mutated into Parasite's familiar "purple people eater" form, after attempting to absorb the powers of Supergirl and Martian Manhunter at the same time. Literally biting off more than he could chew, Jones seemingly became trapped in his monstrous form after absorbing Martian Manhunter's shape-changing powers but not the knowledge of how to control them. Unable to reason with Parasite, Supergirl attacked him with a handful of Plutonium-239, which caused Parasite to explode when he tried to drain the energy within it.
Ignoring the issues that many fans had with how casually Kara Zor-El resorted to killing Parasite in the episode's climax, many felt it was a waste of a good villain to have him die after only a single appearance. Thankfully, the trailer for the upcoming Supergirl season 4 episode "Parasite Lost" reveals that Agent Jensen - a treacherous DEO agent who betrayed the agency to join the anti-alien militia run by Agent Liberty - has become infected by the same kind of alien worm and is seen to be fighting Supergirl and holding his own. Hopefully, this Parasite will last longer than his predecessor and remain a standard-bearer for the rest of season 4.