In the decades following the Roman invasion of Britain in 43 AD, the most powerful native Briton on these islands seems to have been a man named Tiberius Claudius Cogidubnus (or possibly Cogidumnus, or possibly Togidumnus - different versions of the name appear in different sources). We don't know the date of his birth, or of his death; who his parents were; or whether he had any children. His name, however, suggests that he was a Roman citizen (a privilege almost certainly conferred on him by the Emperor Claudius), and an inscription names him as "Great King of the Britons," a title not recorded as having existed either previously or subsequently.
Tacitus, in his biography of the Roman governor, Agricola, tells us that "certain territories were given to Cogidumnus, who lived down to our day, a most faithful ally. So was maintained the ancient and long-recognised practice of the Roman people, which seeks to secure, among the instruments of domination, even kings themselves."
This inscription from Chichester records him as having established a temple there.
To Neptune and Minerva, for the welfare of the Divine House, by the authority of Tiberius Claudius Cogidubnus, Great King of the Britons. The Craftsmens' Guild, and those in it, gave this temple at their own expense. ...ens, son of Pudentinos, presented the precinct.
This is the sum total of what we actually know about the man, although the association with Chichester opens up the possibility (I would say the likelihood) that he was the man for whom the Fishbourne palace was built. If so, he might well have been the heir to a local king named Verica (or Bericus), who fled to Rome shortly before 43 AD, under threat from a more powerful tribe, the Catuvellauni, who had begun invading the territory of the Regnenses tribe (or Regni) from the north. Verica's defection may, indeed, have provided the pretext for Claudius's invasion.
According to this theory, Cogidubnus's grandfather may have been a man named Commius, who minted coins at Silchester in the late 1st Century BC, and his great-grandfather may have been the earlier Commius who was, at least for a time, an ally of Julius Caesar.
There is an alternative theory that "Cogidubnus" may be the same man as "Togodumnus," a Catuvellaunian prince (brother of Caratacos and son of Cunobelinus) who is recorded as having fought against the Romans in the early days of the invasion. The names are sufficiently close to make this credible, but at least one source (Cassius Dio's Roman History) tells us that Togodumnus fell in battle. It does not, in any case, seem entirely likely that the notoriously paranoid Claudius would have appointed as his vassal a man who had raised arms against him. It is a thought worth bearing in mind, however, that a man who somehow had one foot in the pro-Roman Regnensian camp and another in the defeated Catuvellaunian dynasty, might have been particularly attractive to Claudius as someone potentially able to unite the newly acquired province under Roman control.
One of the few certainties is that Cogidubnus, whoever he was, was thoroughly pro-Roman. From the point of view of a novelist, it would be easy to portray him as a traitor motivated largely by personal ambition and greed. This is, essentially, how he is portrayed (as a minor character) both by Douglas Jackson in Claudius, and by Manda Scott in her short story, "The Last Roman in Britain" (www.historyextra.com/boudica). The real Cogidubnus might, for all we know, have been just such a man, but this would not make for a very interesting protagonist. I will therefore portray him in a very different way: as a man seduced in his youth by everything Rome has to offer - the architecture, the food, perhaps above all the beauty of the written word; as a man who choses peace over war; but also as a man who, as he matures, and comes to understand the true nature of power, becomes increasingly troubled by the compromises he will have to make.