Wendy and Marvin? 
Allow me to explain.
The Super Friends debuted in 1973, and the line-up looked like this.
It was the 5 main heroes, and their sidekicks, the "Jr. Super Friends", Wendy, Marvin, and their pet Wonder Dog. They had no powers, and were just basically hanging out with the Super Friends, with no explanation. They weren't even given last names. Their presence was explained like this:
Their function on the show was to stand around, ask questions, get captured (usually after finding the criminal's hideout by mistake), get rescued by main Super Friends, and then help explain the 'moral' of the episode.Comic book readers tend to look for more depth in their characters, so the characters were given origins and last names, and the story of being trained to be crime fighters. The show was cancelled and aired in reruns numerous times, and in 1976 became a surprise hit at that point. 1977 was going to bring a new season, but writers of the cartoon and comic complained about trying to write stories that featured these non-powered characters, so in 1977, the show was renewed as "The All New Super Friends", with Wendy and Marvin gone with no explanation, replaced by the shape-shifting Wonder Twins.
In the comic book, a story was done where Wendy & Marvin graduated to crime-fighter status, but opted to go to college before starting careers in crime fighting.
Here is the original opening for the cartoon from 1973.
