You can headcanon your Champ in a lot of ways and the game's not likely to contradict it (or support it as the case may be) unless your headcanon diverges radically from the background description. So yeah, something like 'my Champ was raised by a barbarian tribe but wasn't born into it' isn't likely to be contradicted by anything that the game is ever going to do. 'My Barbarian!Champ's family is alive' on the other hand would be the sort of thing where you will get contradicted because one of the things that the writers have to distinguish that background from the others to add flavor is 'the rest of the tribe is dead'.
The game never really cares what your Champ is wearing and while dressing like the Kervus is probably more what the writing has in mind I doubt it's ever going to come up if you want to imagine they dressed differently in the past and it's certainly not going to come up regarding how they dress in the present-day.