Warning! Spoilers for The Boys comic and Amazon series below.
In The Boys universe, the most commonly-known speedster is A-Train. However, before A-Train joined The Seven, another speedster was part of the team: Mister Marathon. But, the older speedster's time with the team ended in a gruesome fashion as he had his head decapitated thanks to an accident by Homelander.
In both The Boys comic and Amazon series, Mister Marathon was a part of the original formation of The Seven. While what happened to him on the wildly popular streaming services has yet to be revealed, besides the fact he was replaced by A-Train, his comic fate was extremely dark. Mister Marathon was The Seven's answer to the Flash, as he fought alongside Homelander, Black Noir, Queen Maeve, The Deep, Jack From Jupiter, and Lamplighter. In The Boys #19-21 by Garth Ennis and Darick Robertson, Mister Marathon's bloody last mission is explored.
The Seven's darkest mission was retold in The Boys #21, where it's revealed to Hughie that the superteam had a disastrous rescue attempt of a plane hijacked by terrorists on 9/11. Much like the show's rendition of the plane crash, Homelander's ego and anger get the best of him. He bursts the eardrums of passengers by yelling at them to shut up, accidentally kills someone opening the airlock, and after realizing the pilots are dead, Homelander leaves the passengers to die. When he leaves, Mister Marathon grabs onto his leg and tells him he has to do something. In a heated moment, they decide to try to redirect the plane by slingshotting the speedster against its tail. The plan doesn't work and Mister Marathon is immediately decapitated.
The plan is a massive failure as the ship (now with the tail sliced off) crashes into the Brooklyn Bridge. Everyone on board dies. The Seven's efforts to stop the attack end up with every passenger and one of their own members, Mister Marathon, dying. It's arguably the group's biggest failure in the comics.
Mister Marathon's time in the comic universe was short and ended with a bang. A-Train would later replace him as The Seven's resident speedster and would stick around much longer than the ill-fated Mister Marathon. Could the old speedster appear on the show? Well, technically he already has. In storyboards for The Seven's Dawn of the Seven film, Mister Marathon can be seen as one of the members of the team. His current status is unknown - but at the very least, he didn't die during the plane crash - so it's possible Mister Marathon could still appear on the show.