Oliver Queen / The (Green) Arrow: The Arrowverse version of the Green Arrow really put the character on the map for non-comic readers. He is one of the most popular live action TV superheroes to date, with Stephen Amell receiving critical acclaim for his portrayal. This version of the character has also been included in various video games as downloadable content for Injustice: Gods Among Us and Lego Batman 3: Beyond Gotham with Amell providing the voice.
Felicity Smoak was originally supposed to be just a one-episode tech support agent at Queen Consolidated. But then things happened while shooting that episode and the story was modified to include her in the team. By Season 2, she was a regular and began to replace Laurel Lance as the go-to character for Ship Tease with Oliver. By the third season, she was unquestionably the female lead of the show, especially once she and Oliver became an Official Couple, and in the back half of the series, was elevated towards the top of the opening credits billing, demoting former co-lead Katie Cassidy to an And Starring credit.
Sara Lance / The Canary / White Canary is initially introduced posthumously in Season 1 flashbacks, she was then revealed to be alive in Season 2 using the alias Canary. She was essentially introduced to set up Laurel's Character Development into becoming Black Canary, however she quickly became a Fan Favourite character amongst the fandom with many fans believing she should stay the Canary instead. After the hugely negative reception to her unceremonious death in the first episode of Season 3 (which to many fans felt like a combination of Bury your Bisexuals and fridge stuffing) she was brought back to life and promoted to main characters status in Legends of Tomorrow as the White Canary.
Eobard Thawne / Reverse-Flash: While everyone's performances are generally well-received, it is universally acknowledged that it was Tom Cavanagh's intriguing and multi-layered performance as the likeable but mysterious (and somewhat sinister) Dr. Harrison Wells that really drove most of the show's first season. In fact, he's so good that the showrunners found a way to bring him back for Season 2 and potentially beyond through his Earth-2 counterpart, and in later seasons as his various counterparts from The Multiverse.
Leonard Snart, who would steal the scene whenever he appeared, and quickly became a fan-favorite. Most agree he was also the main selling point for Season 1 of Legends of Tomorrow. It really goes to show how popular he is when he kept getting brought back post-mortem in some capacity, and later permanently via Back Up Twin.
Black Siren, the Earth-2 counterpart of Dinah Laurel Lance. Introduced as The Dragon to the Season 2 Big BadHunter Zolomon / Zoom not long after the death of the original Laurel from Earth-1, she quickly became popular with fans. With her being Truer to the Text compared to the original Laurel and her existence alone being seen as a second chance to write the character properlynote Then Arrow showrunners Marc Guggenheim and Wendy Mericle got a lot of scorn from the comic community and a number of fans for sidelining the character who was so important to source material's mythos, even treating her as a Creator's Pest while also having the potential to be a different character in her own right, many rallied for her to be the Arrowverse's official version of the Black Canary. She's eventually transplanted to Arrow (sort of, as said Earth-1 Laurel was an Arrow main character, meaning her actress just returned to that show) and became a main character there, ultimately becoming the Black Canary towards the end of Arrow Season 7.
This is practically the entire premise of this series: Take a bunch of popular side heroes and villains from the various Arrowverse shows, put them in a team and give them their own show. That being said...it wasn't really until the second season that the show really took off and embraced its own sense of awesome.
From a recurring cast member of Arrow Season 3 to a main character in Legends. Ray has become pretty much synonymous with the series, alongside Sara and Mick. Even after Ray Palmer / The Atom departs from the Legends in Season Five, the writers still managed to find a way to bring him back on The Flash|2014.
Season 2 mixes things up a little bit. While we do get new additions to the team in the form of Dr. Nate Heywood / Steel and Amaya Jiwe / Vixen, this season instead switches things up by introducing Breakout Villains from the Arrowverse as the Big Bads: Malcolm Merlyn / Dark Archer and Damien Darhk from Arrow, and Prof. Eobard Thawne / Reverse-Flash and Leonard Snart / Captain Cold from The Flash.
In a similar way to Tom Cavanagh's Harrison Wells/Eobard Thawne, Maisie Richardson-Sellers's performance as Amaya Jiwe / Vixen became exceptionally popular among both the viewers and the cast members. So despite Amaya being Put on a Bus, Maisie was allowed to remain on set by being given a new character in the form of Charlie / Clotho in Season 4.
Season 3 does a little bit of both. It brings Wally West/ Kid Flash from The Flash as the newest member of the team, and it also brings back Damien Darhk, and lastly one of The Flash's most popular villains: Grodd!
The biggest addition to Season 4 is doubtless John Constantine joining the team, following a few previous guest appearances on Arrow and Legends Season 3. Not only does it bring back John, but all of his baggage as well from the many unresolved plot threads from his cancelled series, enough for many to consider the fourth season being a Spiritual Successor or even a Sequel Series to Constantine (2014).
John Constantine was a largely unknown DC character before this version of him appeared on screens, but his popularity immediately pole-vaulted to fan favorite status. With the cancellation of his show, both fans and the cast looked for a new home and found one by integrating him into the Arrowverse, at first with single episodes guest appearances before he becomes part of the main cast of Legends Of Tomorrow as of Season 4. Ever since this depiction of him, Constantine has been heavily featured in other media including Justice League Dark (with Matt Ryan reprising his role) as well as a part of the cast of Justice League Action.
Some of the show's original characters have become very popular too, like Beebo the Tickle Me Elmo/Furby parody (who eventually rose to Series Mascot status!) and Mr. Parker.
