Christopher Penrose is a serial killer, and the result of Dr. Claus Penrose's experiments to create a rapid aging human. Prior to the Flight 627 attack and the death of John Scott, he and Olivia Dunham had investigated the murders, but failed to identify the killer, whom they nicknamed the "Brain Surgeon".
Christopher kills in order to stop his aging. Starting by giving his victims a muscle relaxant, he then makes an incision in his victim's gums, pulls the gums up to their eyes, and removes the pituitary gland, which controls the body's growth. Consuming the gland slowed down his intensely rapid metabolism for a time, but ultimately, he was not able to keep up with his own altered body, and died of rapid-onset old age. ("The Same Old Story")
An old Christopher Penrose