Raul Castro only came out from under the shadow of his higher profile sibling in July 2006 when health problems forced Fidel to give up power he'd held since taking Havana by storm at the head of a guerrilla army in 1959.
Castro, whose steel-rimmed glasses hide a sharp gaze, was a discreet figure. But as defense minister from 1959 to 2008, he was also perfectly positioned to impose himself as the island's strongman when Fidel faded.
After consolidating control, backed by the military, Castro began small steps toward opening up the sclerotic, Soviet-style command economy.
In parallel, he launched a series of diplomatic initiatives to forge new allies -- notably opening up to the old enemy the United States, where President Barack Obama's arrival in power in 2008 promised a chance for detente.
This shift had practical, as much as any ideological, motivations: leftist strongman Hugo Chavez's Venezuela -- Cuba's main economic partner -- was sliding off the economic rails.
When Fidel Castro stepped down, "Venezuela and Chavez seemed likely to provide Cuba its lifeline for a long time. That's all changed and so has the attitude of the US government under Obama," said Paul Webster Hare, who teaches international relations at Boston University.
Raul Castro, 84, has deepened those diplomatic initiatives, tightening links to the European Union and dramatically responding to the Obama administration's call for an end to their decades-long standoff -- a rapprochement that on Sunday will see Obama making the first trip by a sitting US president in 88 years.
Cautious and with less flair than Fidel, Castro has patiently diversified Cuba's alliances, attracting foreign investment so that an economy still 80 percent under state control no longer relies so much on a single foreign backer.
Yet Castro has given little away of the Communist Party's monopoly on running internal affairs.
"There have been few substantive changes in the heart of the sole party with regards to tolerance for diversity of opinions or for (unsanctioned) opposition groups," said Jorge Duany at the Cuban Research Institute at Florida International University.
Fidel in the background
Most observers believe that Fidel Castro, 89, would not have promoted this realignment had he stayed in power, but that he has still given his blessing from offstage, where he continues to exercise influence.
"It is very difficult to imagine Fidel Castro welcoming a US president to Cuba, but it's important to underline that Raul and Fidel work together and that Fidel recognizes the need for this rapprochement with the United States," Michael Shifter, head of the Inter-American Dialogue think tank, said.
Even before Obama's visit, Fidel put up no resistance to the renewal of diplomatic relations last year and the visit by US Secretary of State John Kerry.
Raul cuts a distinctly different figure to charismatic Fidel, but historians say their partnership is what has got Cuba through so many challenges -- whether the US economic embargo or the devastating collapse of all-important Soviet support at the end of the 1980s.
With Fidel rumored to suffer increasingly bad health and Raul promising to step down in 2018, the Castros now face their ultimate challenge: ensuring that opening to European and US investors doesn't bury their revolution.
"Raul is happy to have the growing US tourism and remittances from Cuban Americans," said Hare. "The blocks will be imposed on political grounds on the Cuban side. That would change the whole narrative of the Cuban Revolution, which Raul wants to preserve."
"Raul's main concern is not to endorse inequality like the Chinese have," said Hare, a former British ambassador to Cuba. "He knows that the revolution would not survive long."
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