As is the case with the skill tree points, when you level up as one character, all of the other playable characters also level up, even if you’ve never played as them before.

That being said, you can continue to give your characters better weapons and armor to help make your playthrough easier, giving them better resources to battle enemies with. To do this, though, you’ll need to collect enough resources, like Nth metal, to craft these upgrades.

Source: https://www.distractify.com/p/gotham-knights-max-skill-points