Away from South Beach’s pulsing pool parties and the tycoon enclaves dotting Biscayne Bay, Ryan Breslow, one of Miami’s richest residents—and, at 27, one of the world’s youngest billionaires—balances cross-legged atop a blue beanbag pillow in his humble three-bedroom bungalow right on the border of Little Haiti. Breslow spends most days alone at home.