There is something familiar about his gaze. Standing with a little boy balanced awkwardly on his shoulders, a burning intensity in the young man’s eyes belies a steely resolve. Yet it is hard to imagine that this same clean-cut Argentinian in suit and tie will, 10 years later, be a hero of Fidel Castro’s Cuban uprising, celebrated for his military prowess and Marxist revolutionary zeal. Or that, after his execution in 1967 by the US-backed Bolivian army, and with the help of another photograph – Alberto Korda’s bearded rebel, long hair tucked into a beret with a red star – Ernesto “Che” Guevara will become a Christ-like myth of the left.

The little boy in the photograph is Juan Martin Guevara, Che’s youngest brother. Fifty years after his brother’s death, he has written a memoir in which he intends to “fight this myth and give back to my brother his human face … Ernesto was a man. We need to pull him down off his pedestal … He would have hated being turned into an idol … It is important to understand that he began as a normal, even ordinary person, who became an exceptional person who others can emulate.”

Juan Martin on the shoulders of his elder brother Che in Córdoba, 1945. Photograph: Courtesy Juan Martin

Ernesto Che Guevara was born in Rosario, Argentina, in 1928, the eldest of five children. The family was an unconventional one. Celia de la Serna came from a well-to-do Buenos Aires family. Educated in a convent, she was intelligent and highly rebellious. She was an early feminist and one of the first women in Argentina to wear trousers, smoke and drive a car. Ernesto Guevara Lynch was six years older and proved irresistible. A restless, fun-loving party animal, he was the black sheep of his respectable family, and was a tanguero at a time when Argentina’s national dance was still considered a sordid working-class pas de deux. The two married in haste. Six months later, Che was born.

Celia and Ernesto brought up their children in a bohemian, financially precarious environment. Freedom of thought and action was encouraged. The family moved regularly. The parents’ radically different personalities – she admired discipline and study; he craved adventure and shirked responsibility – made for a volatile relationship. On the phone from Buenos Aires, Juan Martin explains how much of an influence they had on Che.

“My father lived without wanting to be tied down,” he says. “You could say that he was a little bit too free. But that gave us the feeling that we could travel, see the world. My mother was more principled, she had very firm ideas. Che was a mix of the two.”

Juan Martin was born 15 years after Che in Alta Gracia, a spa town in the province of Córdoba where the photograph of him on his brother’s shoulders was taken in 1945. The family had moved there with the hope that the dry mountain air would help Che’s chronic asthma. It did. As his health improved, so his restless energy grew. He came and went, studied to be a doctor but eventually set off on a two-year motorbike trip through Latin America, which became the subject of Walter Salles’s film The Motorcycle Diaries. It was on this journey that his political ideas were forged. He laid the blame for the terrible inequality and poverty he witnessed on the continent at the feet of the US and its imperialist exploitation. (He described the short period he spent in Florida on that trip as “the worst two weeks of his life”.) And he concluded that the only way to redress this inequality was for ordinary people to rise up and seize power.

Che Guevara in 1959. Photograph: Joseph Scherschel/Getty

But whenever Che returned from his wild adventures, there would be a party atmosphere in the family and he proved to be greatly devoted to them. In his new memoir, Che, My Brother, Juan Martin writes that he “treated me like a son”. In the photograph, Che could be his young father. Was this the nature of their relationship?

“There was a 15-year age difference,” Juan Martin says. “I listened to him more than I would have listened to a sibling of the same age. Was he like a father? Perhaps a substitute one. But at the same time, we did things only brothers do. We played football, had fun.”

Juan Martin also writes: “I grew up in the shadow of Ernesto. I could never escape it. Until 1956, I was Juan Martin Guevara. From 1957, I was the brother of the revolutionary Ernesto Guevara, Fidel Castro’s companion and a fearless warrior. And then a legend. His absences saddened me, his death devastated me. I always say that he had to be someone’s brother.”

Has it been a burden?

“I grew up in the light of his thinking. It’s not a weight, a responsibility like that,” he says. “He has been my companion in the sense of the way I think.”

It took Juan Martin 47 years to visit the place in Bolivia where his brother was executed, a classroom in the tiny village of La Higuera. The experience proved a disturbing one – but also a catalyst for him to set the record straight. The area is now a tourist trap, Che’s image exploited on merchandise, the product of a capitalist system he wanted to bring down. When the locals learn who Juan Martin is, “they’re dumbstruck. Christ can’t have any brothers or sisters.” Che has been turned into a saint, his image prayed to, called on to perform miracles. Horrified, Juan Martin vows “to be a conduit for his brother’s ideas and ideals”.

Before his death in Bolivia, Che told a fellow fighter that he considered Juan Martin, of all his siblings, to be his spiritual heir. His other surviving brother and sister, Roberto and Celia, refuse to talk about Che. In fact, Celia accuses Juan Martin of being “too media-friendly” and, he says, “disapproves when I talk about Ernesto”. For her, his death was almost too painful to bear and for years she was in denial that he had been killed. Juan Martin considers her to be the most like Che of all of them, not only in character but also in her beliefs. He hasn’t told her about the book and fears she will never speak to him again when she hears about it. “I will listen to her criticisms of me,” he says.

Che Guevara’s story is an incredible one, but so is Juan Martin’s. He spent more than eight years as a political prisoner in Argentina in the late 70s and early 80s under the rightwing junta, while he and his family are still revered in Cuba because they are relatives of the legendary Che. Does he think the world needs another man like his brother? Is such a person possible?

“Yes, it’s possible. He is an archetype. Do we need someone exactly the same? A guerrilla? No. A man with a beret living up in the mountains? No. But a man with ideas for change, with principles, who won’t exchange them for money and fame and power. Someone with Che’s characteristics. Yes, we need that. The world has not become a better place from when Che was alive. It has become worse. We must fight to make things better.”

• Che, My Brother by Juan Martin Guevara and Armelle Vincent is published by Polity, £20. To order a copy, go to bookshop.theguardian.com or call on 0330 333 6846. Free UK p&p on orders of more than £10, online only. Phone orders min p&p of £1.99.