John Fitzgerald Brennan (called Jack by friends) is a fictional character in the Known Space series by Larry Niven.
He is famous for being the first Human to meet an outsider (extra-terrestrial), the Pak Protector called Phssthpok (transliteration; it's a hiss followed by a clap of the gums). The Belter enters Phssthpok's ship and is transformed into a Protector stage human in the novel Protector. All of humanitys knowledge of the Pak are from this man's retelling of what Phssthpok told him.
As all Belters, Jack has a painting on his space-suit. This makes it easier to distinguish various people in the distances of space, where everybody is wearing what is essentially the same clothes. Jack's design shows Salvador Dalí's The Madonna of Port Lligat. Brennan had two daughters at the time when "Protector" takes place, Estelle and Jennifer, with Charlotte Leigh Wiggs, a professional farming machine repairwoman from Confinement.
Audited April 21, 2002