They were expelled from their first school together. They fought in Cuba’s Sierra Maestra together. And, following their victory over the forces of Fulgencio Batista, they have ruled Cuba together for nearly 50 years.

But although Fidel became a worldwide icon, instantly recognisable by his first name alone, his younger brother, Raúl, has remained a virtual unknown.

Even in Cuba, people know little about the mysterious and deeply private man who has effectively run the country since Fidel Castro underwent emergency intestinal surgery in July 2006.

Now that Fidel has officially stepped down as head of state, the question is being asked: Who is Raúl Castro and how will he rule the country?

Despite their long and close collaboration, the Castro brothers were in many ways polar opposites.

Fidel was an unusually charismatic leader with a strong hold over the affections of many Cubans, despite the many hardships of life under his rule. He has revelled in the limelight, speaking at public events for up to seven hours at a time and, in the 1960s, mastering the global medium of television before virtually anyone else.

Raúl, by contrast, has kept a low profile, seemingly content to live in his brother’s shadow. Still, those who have watched him say he should not be underestimated.

Though the 76-year-old lacks his brother’s charisma, he is a tough and highly-effective operator, able to be pragmatic and, when necessary, ruthless. He controls the three most powerful institutions in Cuba – the military, the security services and Communist Party – and has no real challenger in sight.

“Fidel has been the visionary, but hopelessly disorganised," said Brian Latell, a retired CIA officer and author of 'After Fidel', a biography of the Castro brothers. “Raúl has provided the organisational glue. That's why he has been the one truly indispensable man in the revolution, other than Fidel himself.”

Particularly in the early years, Raúl acted as his brother’s hard-line enforcer, eliminating opponents and earning himself the nickname “The Prussian” for his cold, efficient style.

Photographs from the late-1950s show him as a slight young man, directing summary executions of scores of Batista’s fighters. Throughout the years, he has sent even close friends to the firing squad, men like General Arnaldo Ochoa, a highly-decorated veteran of Cuba’s campaigns in Africa.

A far more pragmatic side of Raúl Castro has been on display since the collapse of the Soviet Union. As head of the armed forces, one of the few well-functioning institutions in Cuba, he oversaw the opening of the economy to foreign tourism, to US dollars and to limited private enterprise.

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