A twin who survived
Tiberius Gemellus was a son of Drusus the Younger and Livilla, and so a biological grandson of the emperor Tiberius. His twin brother, Germanicus Gemellus, died in early childhood; Tiberius Gemellus survived, though his father died — poisoned, it later emerged — when he was still a small child.
Named joint heir, then adopted by a rival
When Tiberius died in 37 CE, his will named Gemellus joint heir alongside his great-nephew Caligula. The Senate and the praetorian prefect Macro instead recognised Caligula alone, who then adopted Gemellus — technically making his own cousin his son, while giving Caligula full authority over him.
Removed within the year
Caligula had Gemellus killed in 37 or 38 CE, reportedly once he felt secure enough in power to no longer need the gesture of shared succession. His death removed the last grandson of Tiberius with any claim on the throne. A marble portrait head at the Musée Saint-Raymond in Toulouse has been proposed as a likeness, though the museum’s own catalogue marks the identification with a question mark.
Historical sources
Suetonius, Caligula 23; Tiberius 76
Tiberius’s will naming Gemellus joint heir, and his death under Caligula.
Tacitus, Annals 6.46
Gemellus’s position at the end of Tiberius’s reign.
Portrait: Christelle Molié / Wikimedia Commons, CC BY-SA 4.0. The museum’s own catalogue marks the identification with a question mark. Resized for display.