Barry Allen is a superhero from comics publisher DC Comics who is the second iteration of the Flash following Jay Garrick. Barry and his various iterations an essential part of the DC mythos, from being a founding member of the original Justice League in 1960, his sacrificial death to save the DC universe in 1986's Crisis on Infinite Earths, his return from the dead in 2008 and role in the defeat of Post-Crisis Darkseid in Final Crisis, his role in rebooting the mainline DC Universe during the Flashpoint story in 2011, and his role in the merging of Post-Crisis and Post-Flashpoint history together in 2016's DC Rebirth and 2021's Infinite Frontier.
Barry was the main Flash of DC's "Earth-One" or "Pre-Crisis" universe from his introduction in 1956 until 1986 when he died in the DC continuity-resetting story Crisis on Infinite Earths in 1986. Prior to his death, Barry's life had fallen apart with his nemesis Reverse Flash killing his wife Iris West, him subsequently being put on trial for murder after killing Eobard Thawne when defending his second wife Fiona Webb (who was driven insane from the ordeal), and being kicked out of the Justice League. Barry later learned that Iris was actually from the future (her birth family, a 31st century family known as the Russells had sent her forward in time to save her life from a war) and alive in a body made by her biological family, which led to Barry retiring to the future to be with Iris but he was only able to spend a month with her before he went back in time to help fight in the Crisis where he died destroying the main weapon of the Anti-Monitor, the villain of the event.
During the Post-Crisis era of DC, Barry's death was preserved as an example of sacrifice amongst his fellow heroes, mainly his nephew and successor as the Flash Wally West, whom had once been Barry's sidekick as Kid Flash but he scarcely actually appeared in stories and became less relevant as the years went by. After Barry's death Iris eventually gave birth to her and Barry's children, a set of twins named Don and Dawn Allen who became a duo of superheroes known as the Tornado Twins. Don's son Bart and Dawn's daughter Jenni also eventually became superheroes, with Bart being sent to the present to cure his accelerated aging by Iris and became the hero Impulse while Jenni became a member of the 31-st century superhero team the Legion of Superheroes. Barry was revealed to not actually be dead but had instead been trapped within the Speed Force after Crisis, with the Speed Force being the source of the powers of speedsters like Barry and returned from the dead in 2008 during the storyline Final Crisis, where he and Wally helped lead the final defeat of Post-Crisis Darkseid by leading the Black Racer, a Grim Reaper-esque Speed Force entity straight into Darkseid. In the few years that he was a part of the Post-Crisis universe, Barry would be established as a member of the Flash Family, the group of speedsters that consisted of numerous Flashes and Flash adjacent heroes including Wally West, Wally's twin children Irey and Jai (who like Bart experienced accelerating aging as a result of being born with a connection to the Speed Force), Jay Garrick, the veteran speedster Max Mercury, the daughter of Golden Age speedster Johnny Quick Jessie Quick, and Barry's grandson Bart. Barry's origin story was also rewritten and his new origin established that his mother had been murdered when he was a child and he had never found the identity of the true killer, with his father being imprisoned for the act. Following his return Barry would uncover that his mother's true killer was his archenemy Eobard Thawne/The Reverse Flash. He also temporarily joined the Blue Lantern Corps as a part of the DC Comics event Blackest Night.
In 2011, Barry was used to reboot the DC Universe with the storyline Flashpoint, where Barry (with additional interference from the Watchmen character Dr. Manhattan messing with the DC Universe) caused another reset of the DC Universe by trying to prevent Reverse Flash from killing his mother before allowing his mother to die to fix the timeline. This time DC's main continuity changed from the Post-Crisis continuity to the New 52 universe. In this timeline Barry was no longer married to Iris, was now a new hero, and no longer had the Flash Family. Eventually, Barry gained a sidekick in Iris' nephew Wallace West, a new version of Wally West.
It was later revealed that Barry was not fully responsible for the rebooting of the DC Universe, as while Barry had caused a new timeline to be created, characters like Wally West no longer existed due to the meddling of Dr. Manhattan, who had removed 10 years from this new timeline. The New 52 Wally West was re-worked to be an entirely different character, with both the new version of classic Wally West and New 52 Wally West being nephews of Iris and Barry who were both named after a great-grandfather and New 52 Wally now goes by Wallace West or Ace as Wally nicknamed him. Barry, Wally and Wallace also gained an ally in Avery Ho, a teenager who gained a connection to the Speed Force after the Speed Force Storm of Central City. After the events of the Infinite Frontier story in 2021, Jay, Jai, Irey, Max, Jessie, and Bart were restored to DC continuity and united with Barry, Wally, Wallace, and Avery to form the current iteration of the Flash Family. Barry would be the main Flash of the DC Universe from 2011 to 2021 and concluded this second term as the main Flash by finally defeating his archnemesis Reverse Flash in the story Flash: Finish Line by forgiving the villain for all he'd done to him and sending Thawne to a timeline where he'd never become the Reverse Flash, with Wally West returning as the main Flash after Eobard's final defeat. Barry remains active as the Flash and is both a supporting character in main Flash stories and a main character in teams like the Justice Incarnate and Justice League.