Author George MacDonald Fraser done outright stole Flashman from Thomas Hughes' Tom Brown's School Days, a book from 1857 you probably didn't read but a lot of Brits did in the 1960s. For Hughes, Flashy was a coward and a bully, expelled for drunkenness; Fraser spun out his career after that. Yes it's fan-fiction. No it's actually good.
"Harry Paget" wasn't what Hughes had named him. That name came from the cad who'd straight-up sent the Duke Of Wellington's brother to the cuckshed. Under Fraser's hand Flashman will blossom into an adventurer despite himself over... well, over EVERYWHERE.
The first Flashman was pretty much a satire of Victorian heroic fiction about the Northwest Frontier, tracking hard with Rudyard Kipling. Over fifty years of novels Flashman will show up in almost every major world event from the fall of British Afghanistan in the 1840s to Custer's Last Stand in 1876. Usually with his pantaloons spattered with dysentery as he tries to get far far away.
Fraser sold some film rights, which didn't happen, but he pocketed the cash. Then he sold more film rights to the sequel Royal Flash which did happen, in 1976. Fraser spent a lot of time in the Isle of Man swimming in massive piles of British sterling like Scrooge MacDuck.
As for why /tg/ cares - Flashy ranges over settings and gets into scrapes - mostly historical - you'd barely believe ever existed or happened, but they did. Sandy Mitchell based his Ciaphas Cain on this template (with a touch of Blackadder) in the Warhammer 40000 setting.
Seriously, if you haven't read Flashy, why the fuck are you even here. Get off this site and get started on it.