Athina Roussel may one day control the billions amassed by her grandfather, Greek tycoon Aristotle Onassis, but when it comes to getting engaged, the press-shy heiress is anything but ostentatious. This month, word leaked out that Roussel, 20, plans to wed her boyfriend of two years, 32-year-old Brazilian equestrian Alvaro Affonso de Miranda Neto—"Doda" to pals—who proposed after Christmas. "We're engaged," confirms Miranda, who lives with Roussel in his hometown of São Paulo. "We just haven't worked out all the details of the wedding," to be held, he says, this year. Why the secrecy? Pals say Roussel doesn't relish the sort of paparazzi blitzkrieg that accompanied Ari's nuptials to Jackie Kennedy on Skorpios. The pair broke the news to loved ones with "little fanfare," says a source close to the clan. "She doesn't like to fuss."

The son of a wealthy businessman, Miranda shares Roussel's passion for horses and is described as "down-to-earth" by Silvia Milani, spokeswoman for the club where both ride. He also happened to be ending a relationship with model Sibele Dorsa—with whom he has a daughter, now 5—when he met Roussel at a stable in Belgium. While Euro tabs have suggested that dad Thierry Roussel, Christina Onassis's fourth husband (he remarried two years after Christina died in 1988), disapproved of the match, Athina didn't falter. As Miranda's mother, Beth, puts it, Athina "is very mature."

Will it be enough to break her family's bad luck at the altar? Something Miranda told PEOPLE in 2003 gives reason for hope. "I love her with all my heart," he said then, "and I think she feels the same way about me."