Although the origin of Plastic Man was never related on the show, his name is Patrick "Eel" O'Brian, and he was a thief who broke into a plastic factory with a few of his friends for a heist and was doused in experimental plastic.
Plastic Man is actually a rubber man. He got his name because there was already a hero owned by MLJ (now Archie Comics) called 'Flexo the Rubber Man' at the time that Plastic Man made his debut in the 1940s.
In the cartoon, Penny and Hula-Hula were Plastic Man's sidekicks. In the comics, however, his sidekick was Woozy Winks.
When Plastic Man ran in syndication, Mark Taylor (as Taylor Marks) portrayed the title character in live action segments.
Plastic Man and Superman were not the only preexisting superheroes Ruby-Spears sought to adapt. Jack Kirby helped produce a pitch to make a Thor animated series in 1981.