It tracks. If you look at your toons at 55, Luke and IG-90 both are skill 7 in abilities and specials and 65 translates to gear 12. If you level your toons 1 level at a time and keep them equal, you won't run into an issue with either or outpacing each other and screwing up your Coliseum run being underpowered and underpowering your borrowed units. My Luke and IG-90 are both 26, I am on tier 3 in arena and I will continue to level all my toons equally so they are all viable. I have bought nothing other than the era pass and one pack for currency. I am scoring in the 60-100 rank in Coliseum now since everyone over-leveled their Luke compared to IG-90. Mine are on parity with each other.