The blonde waitress wearing a Legion of Superheroes flight ring in the Young Justice season 3 finale is one of the biggest mysteries going into season 4, and it's possible she's actually Saturn Girl. The identity of this mystery woman has proven as intriguing to fans of both the classic superhero team and the popular DC Universe animated series as the question of what would prompt a Legion member to travel back in time to the 21st century, though it will doubtlessly be a major part of the story of Young Justice season 4.
Young Justice season 3, episode 26, "Nevermore," ended with the founding members of The Team meeting for a private party at Bibbo's Diner in Metropolis. The heroes and their families were waited on by a blonde waitress, whose face was never shown as the scene continued. The episode ended with a close-up on the waitress' hand, as she refilled the coffee mug of Miss Martian. This revealed her to be wearing the trademark ring of the Legion of Superheroes.
First appearing in Adventure Comics #247, the Legion of Superheroes were the first team of superheroes in comic book history to be made up entirely of teenagers. Based on Earth in the 30th century of DC Comics' future, the Legion served the United Planets as a peacekeeping force, recruiting young people with special abilities from across the galaxy. While the team's membership has varied over the years throughout a number of reboots, a core line-up of Legionnaires has consistently remained as the heart of the team.
Considering that core, it seems likely that the mystery Legionnaire is Imra Ardeen, aka Saturn Girl. A native of Saturn's moon, Titan, Imra is a powerful telepath and telekinetic. She was also the sole female member of the Legion's leadership when the team first appeared in 1958. The chief reason to suspect Imra is the waitress is her physicality. The waitress appeared to be a Caucasian woman with long blonde hair pulled back in a pony tail. While her hairstyle has varied over the years, one of Saturn Girl's most popular designs featured her long blonde hair pulled back in a pony tail while she was at work. Also, up until her appearance in Supergirl season 3, Saturn Girl had always been portrayed as a blonde Caucasian woman.
Ignoring her prominence and popularity among fans of the Legion of Superheroes and the wisdom of giving her a key role in introducing the team into the reality of Young Justice, Saturn Girl's powers also makes her the ideal advance agent for a Legion mission into the past. Saturn Girl's telepathy would allow her to gain the knowledge she needed to convincingly pass as a 21st Century native and erase any memory of her existence. Even if the Legionnaire is someone else, fans are still left to wonder just why the Legion would need to spy on the heroes of the 21st Century and how many members of the team might have made the journey along with the blonde waitress.