Your main rival is Kaz, when you pick a starter dragon, he'll pick a different one that you didn't pick. We have plans for the 3rd dragon that didn't get picked much later on. Each of your friend rivals, Pete/Dray/Florina & somewhat enemy rivals Sydney/Kayo all have Dragons as their main pokemon. It'll be explained why much later on as well. Other then Kaz, the rest of the names mentioned will always have the same pokemon no matter which dragon you pick at the beginning.
Since Dray/Florina are from Sinnoh/Unova, you can expect their pokemon to be from there.
Sydney is more focused on attacks while Kayo has strategy mixed with power.
V is all about status affects and is the most annoying enemy to battle.