The presumed author of the Historia Brittonum (History of the Britons), the prologue to which identifies the author as one Nennius or Ninnius, disciple of Eluodogi or in the Welsh Elfod. Elfod or Elfoddw was certainly a bishop of Bangor, in the kingdom of Gwynedd, in 768 AD, and is recorded as dying around 810 AD.
There is also a reference to a Romano-British cleric called Nemniuus in an early 9th century Welsh manuscript.
All of which gives some historical credence to the assertion that there was a Nennius in the early 9th century who could have compiled the Historia Brittonum. None of which has prevented the contrary assertion that the authorship of Nennius is an entirely late medieval concoction.
If there was such a person he must certainly have been a cleric but other than that we can say nothing of his life. Suggestions that he was an official interpreter or an envoy for the royal court at Gwynedd are merely speculation and have no real historical basis.