It varies from faction to faction. For the Boii we have historically attested individuals who were living at that time and so we made use of these. In the case of others "Celtic" we worked on the idea, which is current in archaeolohical theory, that such societies owed to their ethonym to a semi-mythical founder. So for example the Averni (as far as I remember) start off with Aruernos as their faction leader.
But yes, much of it is educated fantasy when it comes to creating family trees. Although saying that, even if you were to play a game in EBII which was historically accurate to the nth degree, it is unlikely that, invading Gaul in 58BC, you would find the Aedui being led by Dumnorix and Diviciacus etc.
Bookmarks