This article contains one or more spoilers. This is a potentially bad thing. Be careful reading on if you are not well-versed in the continuum or not caught up!
The Mysterious Somebody was the villainous leader of the PPC from 1992 to 1999, and subsequently ran the Mary Sue Factory. He took over the PPC by inducing an epidemic of insanity, was ousted during the Reorganisation, and was ultimately killed during his invasion of HQ in 2006. He is written by Huinesoron.
The Mysterious Somebody generally came across as an amiable sort, someone you could easily trust. He made frequent use of his mind-control powers to ensure that no-one realised that they had no actual reason to trust him.
Before the PPC
The Mysterious Somebody began life as a bit part in a Star Wars bad slash story: a clone of the insane Dark Jedi Master Joruus C'Baoth. When Agents Anya, Josephine, and Suzay came to spork his fic of origin, he used his mind-control powers to persuade them to bring him back to Headquarters. He refused to give his name.
The Mysterious Somebody joined the PPC in 1992. After a short time working as a secretary in the Disturbing Acts of Violence Department, he instigated an insanity epidemic in Headquarters that led to the death of the Poppy, the abdication of the Sunflower Official from his position as Head of the PPC, and the Somebody's own takeover of the organisation.
Head of the PPC
He established the Department of Internal Security after his rise to power, and the early years of his reign were a golden age where wages were high and morale was higher; unknown to most of the PPC, much of the organisation's wealth came from PPC Bioengineering Inc and the original Mary Sue Factory, set up under the Somebody's watch, and the DIS were entrusted with keeping this secret at any cost.
After the PPC
Seven years later, he was deposed after Dassie Hyrax, briefly freed from the Somebody's mind control by the presence of Luke Skywalker in HQ, identified him to the Sunflower Official in the events of the Reorganisation. After this event, he would use the Sue factory to produce an army of Sues, planning to lead them in an attack on HQ. The attack occurred at the same time as an assault by the Black Cats, and both sides already in HQ took heavy casualties from the insane clone. His army interrupted a battle in the Large Auditorium, decimating DAVD and nearly wiping out Parma Division despite their combined efforts, shortly after which he went off on his own and left a clone of Dassie in charge of the main force. He was later killed by the Sunflower Official and Lady Zhevago, who caused him to be hit with his own Force Lightning. At this point, any members of his army who had not already died in the battle outside the Tomb of the Unknown PPC Agent dropped dead, unable to survive without his mental control.
The Mysterious Somebody's mirror counterpart is probably still alive.