I think youre thinking about combos! Each hero has a number of tags. Normal has 1, Rare has 2, Epic has 3. They do fun stuff like creating fire trails, letting Claudia have two clouds instead of one, etc.
To activate a combo, what you need to do is recruit heroes who have the tags the hero needs. The heros own tags dont count towards the unlock.
If you want to see all the tags you have available, you can see it on the left side of the screen, either when the character details are open or when you hover a hero.
We are considering implementing a level up system in the future :)