Frenchie's real name and his new origin story were revealed in The Boys season 2, episode 6, which also showed the team's history with the supe Lamplighter. The Boys season 1 teased an unspoken event that broke apart the team and forced CIA Deputy Director Grace Mallory into retirement. That event ultimately made it suicidal to go after the supes, according to Mallory's successor, Susan Raynor, who was killed for getting too close to Vought's plans in The Boys season 2 premiere.
It's still unclear who killed Raynor and how, but one mystery was solved regarding the night Lamplighter "torched" Mallory's grandchildren. The Boys season 2, episode 6 revealed that Mallory and the Boys attempted to blackmail Lamplighter into giving them secrets about Vought, The Seven, and specifically, Homelander. Lamplighter reluctantly agreed, but then later that night, he went to Mallory's home and burned her grandchildren alive. The thing is, he thought Mallory was in the bed - or was supposed to be. So while he still attempted to murder Mallory, he carried the guilt of killing her grandchildren for years.
Frenchie was tailing Lamplighter that night and should've been there to stop him, but his girlfriend Cherie needed his help to save their friend, Jay, who was overdosing on drugs. When Cherie called Frenchie and he refused to leave his post, she used his real name, Serge (his last name is still unknown). That name was first mentioned in the character's dossier that was posted to Amazon Prime Video's official landing page for The Boys season 2, but it wasn't until episode 6 that it was used on-screen.
The night Lamplighter killed Mallory's grandchildren and Frenchie saved his friend happened five years before the events of The Boys season 2. But by that point, he had already spent three years working for Mallory and her team, which also consisted of Butcher and Mother's Milk. In 2012, Frenchie and his friends were arrested in a botched bank robbery, which is when Mallory found him and offered him a get-out-of-jail card; his friends would go free if he came to work for her, using his abilities to find ways to counteract supes' powers. And while an intriguing origin story, this version is starkly different from the character's comic book origin.
In the comics, Frenchie aka The Frenchman was recruited by Butcher about one year after he killed the supe Black Pierre, who was not only Frenchie's enemy but someone his the love of his life was in a relationship with. After killing Black Pierre, Frenchie left his home and wandered around until he was found by Butcher. While Frenchie's origin in the Amazon Prime series is notably different, the new story makes more sense when looking at the bigger picture. Mallory was their leader; she brought Butcher into the fold, so it's only reasonable that she did the same for Frenchie too.