One of the big DC Comics teams that the Arrowverse has added is the Legion of Super-Heroes, with several members confirmed to exist in The CW's DC TV universe. When Arrow began in 2021, that was the beginning of the Arrowverse, which has become one of the biggest comic book live-action universes of all time. One of the common aspects of the franchise is the leading hero always having a big team around them. From Team Arrow, Team Flash, the Super-Friends, the Legends, the Outsiders, and the Bat-Family, the titular hero never has to be on their own.
On Stargirl, Courtney Whitmore has revived the Justice Society of America with new heroes taking up the mantles of the Golden Age team. Superman & Lois is the biggest exception to the rule as Clark Kent works primarily with Lois Lane and the Department of Defense. While the Arrowverse has formed its own Justice League, that isn't the only iconic DC team to exist within the franchise. During Supergirl season 1, there was a quick glimpse of a Legion ring in the Fortress of Solitude, teasing the Legion of Super-Heroes perhaps showing up one day.
But by Supergirl season 3, the Legion of Super-Heroes officially made its way into the Arrowverse. This became the second live-action iteration of the futuristic team, as the Legion appeared in Smallville season 8. Multiple members have emerged from page to screen throughout Supergirl's run, either through physically appearing or being name-dropped by their fellow Legionnaires. With Supergirl coming to an end after the sixth season, it is likely that will also be it for the Legion's regular presence in the Arrowverse. But that doesn't change the fact of how many members actually exist within the world of the Arrowverse.
Mon-El
After Chris Wood's Mon-El accidentally got sucked into a wormhole in the Supergirl season 2 finale, it became the beginning of the Legion of Super-Heroes. Sometime after Mon-El got to the 31st century, he became the founder of the super-team, using Supergirl as an inspiration to create a force for good. The Legion consists of heroes that come from all over the galaxy, bringing several alien species together to fight evil. While Mon-El returned in the third season, he ultimately goes back to the future to continue leading the Legion of Super-Heroes.
Brainiac 5
One of the most iconic Legion of Super-Heroes members in the comics is Querl Dox, a.k.a. Brainiac 5, a relative to the primary Brainiac. In Supergirl season 3, Brainiac 5, played by Jesse Rath, is one of the new members to join Mon-El in the 21st century while they were stranded there. But by the end of Supergirl season 3, Brainiac 5 is forced to stay behind, making him a regular Arrowverse player. It wasn't safe for Brainiac 5 to return to the 31st century due to a particular evil relative having unleashed a plague that could kill every A.I. in existence, except for the criminal relative, who is almost undoubtedly meant to be Brainiac.
Saturn Girl
Supergirl season 3 also saw the Arrowverse arrival of Imra Ardeen, a.k.a. Saturn Girl, another huge Legion of Super-Heroes member. Played by Amy Jackson, this version of the Titanian heroine becomes Mon-El's wife after he arrived in the 31st century. While they aren't married anymore in the future, Imra was one of the more recognizable Legion members to join the Arrowverse. Throughout her time in Supergirl season 3, Saturn Girl was one of the stronger heroines, given her psychic abilities.
Toyman
When Brainiac 5 had to stay behind in the past, Jeremy Jordan's Winn Schott took his place and joined the Legion of Super-Heroes. When he returned for a two-parter in Supergirl season 5, titled "Back to the Future," Winn revealed what had happened to him in the future. Not only did Winn get married and start a family, but he had become known as Computer Lad. But after stopping his evil doppelganger, Winn took on his father's moniker Toyman, hoping to turn that name into something good after all the pain Winslow had caused as the super-villain.
Chameleon
While there have been a handful of on-screen Legion of Super-Heroes, the Arrowverse have acknowledged other members that exist in the future. In Supergirl season 3, episode 18, titled "Shelter from the Storm," Reep Daggle, a.k.a. Chameleon, is name-dropped as a Legionaire who got severely sick thanks to the Blight. In the comics, Chameleon, a shape-shifter, comes from the planet Durla, the same planet where Lynda Carter's Olivia Marsdin was born. But after Supergirl and the Legion stopped Pestilence in the past, the future never got affected by the Blight, including Chameleon.
Chemical King
During Winn's return in Supergirl season 5, the Chemical King is referenced in "Back From the Future - Part Two." While little is revealed about the Arrowverse iteration, it is established that he had chemical-related powers. In the comics, Condo Arlik, a.k.a. Chemical King, was a Phlonian, who had the gift to control chemical reactions. However, unlike other Legion members that have been confirmed to exist in the Arrowverse, Chemical King is one of the minor ones who haven't appeared in comics for decades.
Nura Nal
While most Supergirl viewers may know of Nicole Maines' Nia Nal as an original Arrowverse creation, she is heavily based on the DC heroine Nura Nal. But the Arrowverse actually made them related, with Nia being Nura's ancestor. As Nia operates as Dreamer, Nura is known as Dream Girl in the comics and assumingly in Arrowverse's 31st century. Nura has pretty much the same Naltorian abilities as Nia where she can see the near future through her dreams. Whether or not fans will get to see Dreamer and Dream Girl meet each other before Supergirl ends remains to be seen.
Lightning Lass
In "Back From the Future - Part Two," Winn references a Legion of Super-Heroes member named Lightning Lass, which is Ayla Ranzz's superhero name in the comics. In Supergirl's future, Winn got married to Ayla, who comes from the planet Winath, and has electric-based powers. Ayla was referenced even in Supergirl season 3 as one of the Legionnaires who got affected by the Blight, which had consumed her home planet.
Lightning Lad
Ayla isn't the only Ranzz to have joined the Legion of Super-Heroes in the comics, as there is also Garth, a.k.a. Lightning Lad. In Supergirl season 5, while not referenced by name, Garth is alluded to being Winn's brother-in-law and the uncle of Ayla's daughter. But in The Flash season 5, Lightning Lad is also referred to as a hero who exists in the 21st century's future and has a lightning bolt as his superhero emblem too. But post-Crisis, there is definitely only one Lightning Lad in the Arrowverse, who is from the 31st century.
XS
Supergirl is primary where the Legion of Super-Heroes is used, but before Crisis on Infinite Earths, it seemed like Earth-1 had one too in the future. In The Flash season 4 and 5, Jessica Parker Kennedy played Nora West-Allen, a.k.a. XS, who is Iris West-Allen and Barry Allen's daughter from the future. Nora is actually the one who references Lightning Lad in the season 5 premiere, alluding they worked together in her timeline. Despite Nora being partially an original Arrowverse creation, she is actually based on two DC characters. One of them is Dawn Allen, Iris and Barry's canonical daughter, who has a twin brother named Don Allen.
The other DC player Nora is loosely based on is Jenni Ognats, the original hero who uses the XS mantle in the comics. In the DC Universe, XS, who was Iris and Barry's future granddaughter, was actually a member of the Legion of Super-Heroes. The Arrowverse even confirmed that Kennedy's XS was a Legion member too. In The Flash's 100th episode, titled "What's Past is Prologue," Gideon recognizes Nora as XS, "the fifth recruit in the rebooted Legion of-" before being cut off. But this confirms that not only did a Legion of Super-Heroes exist in Earth-1's future, but the futuristic team would at some point be revived and have XS as one of its core members.
But after the timeline changes in The Flash season 5 and Crisis on Infinite Earths, it is unclear if the Legion is ever rebooted. However, since XS will be returning in The Flash season 7, perhaps that future still exists for her where she does join the Legion of Super-Heroes. While it would have been fun if the Arrowverse ever did a big Legion of Super-Heroes gathering, it is understandable why that may have been hard to pull off. But who knows, perhaps Supergirl season 6 will have a big Legion of Super-Heroes assemble moment as a surprise before it ends its run in the Arrowverse.