Gaius Velleius - Main Biography

Could it be:

Paterculus' grandfather Gaius Velleius had been prefect of the military engineers (praefectus fabrum) in the army Pompey the Great, and later in that of Marcus Brutus, the man who had killed Julius Caesar in 44 BCE and tried to restore the Roman republic. At Philippi in Macedonia, in 42, this army was defeated by Mark Antony and Octavian, the new leaders of the Caesarian party. Velleius was probably present, and he now sided with his powerful personal friend Tiberius Claudius Nero. During the next year, Nero got involved in the war waged between Octavian and Nero's friend Lucius Antonius (a brother of Mark Antony). Velleius was unable to help his protector, and may have seen his position in the Campania threatened, so he committed suicide.note

Octavian defeated Lucius Antonius, but was clement towards Claudius Nero. Perhaps he had already fallen in love with the Nero's wife Livia. In 38, she remarried, and Octavian adopted her sons Drusus and Tiberius. The latter was to play a very important role in the life of Velleius Paterculus.

Velleius Paterculus - Livius

Quote

I shall not deprive my own grandfather of the honourable mention which I should give to a stranger. Gaius Velleius, chosen to a most honourable position among the three hundred and sixty judges178 by Gnaeus Pompey, prefect of engineers under Pompey, Marcus Brutus, and Tiberius Nero, and a man second to none, on the departure from Naples of Nero, whose partisan he had been on account of his close friendship, finding himself unable to accompany him on account of his age and infirmities, 2 ran himself through with his sword in Campania.

LacusCurtius • Velleius Paterculus — Book II, Chapters 59‑93