Postumus Iunior was an usurper

Roman usurper

Usurpers are individuals or groups of individuals who obtain and maintain the power or rights of another by force and without legal authority. Usurpation was endemic during roman imperial era, especially from the crisis of the third century onwards, when political instability became the rule.The...

 against Roman Emperor

Roman Emperor

The Roman emperor was the ruler of the Roman State during the imperial period . The Romans had no single term for the office although at any given time, a given title was associated with the emperor...

 Gallienus

Gallienus

Gallienus was Roman Emperor with his father Valerian from 253 to 260, and alone from 260 to 268. He took control of the Empire at a time when it was undergoing great crisis...

, according to the Historia Augusta. He is included in the list of the Thirty Tyrants

Thirty Tyrants (Roman)

The Thirty Tyrants were a series of thirty rulers that appear in the Historia Augusta as having ostensibly been pretenders to the throne of the Roman Empire during the reign of the emperor Gallienus....

.

According to the often unreliable list of Thirty Tyrants, the Emperor of the Gallic Empire

Gallic Empire

The Gallic Empire is the modern name for a breakaway realm that existed from 260 to 274. It originated during the Roman Empire's Crisis of the Third Century....

 Postumus

Postumus

Marcus Cassianius Latinius Postumus was a Roman emperor of Batavian origin. He usurped power from Gallienus in 260 and formed the so-called Gallic Empire...

 had a son, also called Postumus, whom he nominated first caesar

Caesar (title)

Caesar is a title of imperial character. It derives from the cognomen of Julius Caesar, the Roman dictator...

, and later even augustus

Augustus (honorific)

Augustus , Latin for "majestic," "the increaser," or "venerable", was an Ancient Roman title, which was first held by Gaius Julius Caesar Augustus , and subsequently came to be considered one of the titles of what are now known as the Roman Emperors...

and co-ruler. Postumus Iunior should have been killed together with his father, during the rebellion of Laelianus

Laelianus

Ulpius Cornelius Laelianus was a usurper against Postumus, the emperor of the Gallic Empire. His revolt lasted from approximately late February to early June 269.-Origins:...

 (called Lollianus by Historia).

The author(s) of Historia tells that Postumus Iunior was a skilled rhetor, and that his Controversiae were included among Quintilian

Quintilian

Marcus Fabius Quintilianus was a Roman rhetorician from Hispania, widely referred to in medieval schools of rhetoric and in Renaissance writing...

Declamationes.