Romero 'Romeo' Parada is a former Mexican military commando, and now a high-ranking member of a Galindo Cartel. He is one of SAMCRO's new gun buyers. At the end of season four is revealed that Romeo is actually an undercover CIA collaborator.
It is revealed in the second episode of season four that the Sons' alliance with the Mayans was for more than guns, it was for protection in Stockton. The Mayans' connection to Parada's cartel was what kept the Sons safe from the Russians in jail. His cartel has such influence in the California underworld that most of the Hispanics in prison protected the SOA.