Hector Xavier Monsegur (born 1983), known also by the online pseudonym Sabu (pronounced Sə’buː, Sæ’buː), is an American computer hacker and co-founder of the hacking group LulzSec. He later turned informant for the FBI, working with the agency for over ten months to aid them in identifying other hackers from Lulzsec and related groups. LulzSec intervened in the affairs of organizations such as News Corporation, Stratfor, British and American law enforcement bodies and Irish political party Fine Gael.
Sabu featured prominently in the group’s published IRC chats, and claimed to support the ”Free Topiary” campaign. The Economist referred to Sabu as one of LulzSec’s six core members and their ”most expert” hacker. More Info here