Long story short, the 20th Century's most widely-known British non-commissioned officer was real. Only his name wasn't Pepper, it was Babington. And he was a Lieutenant General.
Paul McCartney chose the image of Gen. Sir James Melville Babington as the real-life visage of the fictional Sgt. Pepper for the Beatles 1967 album Sgt. Pepper's Lonely Hearts Club Band. For most people, being on a Beatles album would be the highlight of their life. Not so for one of the British Empire's decorated officers.
The Scottish-born Babington came up in the ranks of the British Imperial military through the Boer War of the 19th century, spending decades fighting insurgencies against the Dutch descended residents of the southern tip of Africa. He scored a number of decisive wins there, becoming a feared opponent of the rebels. He left just before the end of the war, which went just about as well as you think it might when a bunch of farmers take on the largest empire on earth.
After laying the smack down on the Boers in South Africa, he did a brief stint in England before being transferred to take command of the New Zealand Defence Force in 1902. After five years, he was sent back to London, where he stayed until World War I broke out.
From there, he took command of the British 23rd Division under the New Army. Described as "elderly but fearless" he spent a lot of effort and Crown funds on outfitting his men, unlike many other commanders. As a result, his men loved him and fought so hard at legendary WWI battles like the Somme and Ypres. He also led men along the fronts that aren't as talked about in history books, like Italy and the Asiago Plateau.
When he retired, he was Lieutenant General Sir James Melville Babington KCB, KCMG, commander of British Forces in Italy. He died in 1936, and would never know that his face finally achieved worldwide fame, probably even in South Africa.