Galba records speculation that his father was a gladiator named Martianus, whom he resembled, but it was rumored (probably falsely) that he was the illegitimate son of a previous Roman Emperor, Caligula.
Nymphidius gradually consolidated some authority over the Praetorians, and his promise of material reward increased their willingness to declare for Galba during the provincial revolts of 68 which led to the senatorial rejection of Nero and to his suicide that same year.
Between the death of Nero and the arrival of Galba at Rome, Nymphidius wasted no time: he orchestrated the "resignation" of Tigellinus and stood as sole commander of the praetorian guard.
Galba, however, appointed a replacement for Tigellinus, Cornelius Laco, and took several further steps to eliminate potential rivals (e.g., the murder of Lucius Clodius Macer in Africa Province), all of which must have made Nymphidius uneasy.
No longer content to help others to the throne, Nymphidius declared that he himself was a legitimate successor to Nero — a claim which he supported with the dubious assertion that he was the illegitimate son of the former emperor Caligula.