Several champions have collections or "families". Within these groups, each champion can be common, rare, or legendary, with different abilities. Remember that some champions are unique, having no collections.
You can check the different champions when accessing the champion menu, by scrolling left or right. In the upper-left corner, you can check which level that champion is currently in, alongside with the maximum level it can reach. On the bottom, you can check how you can get that specific champion and how many skills are already leveled up.
Different rarities have different maximum levels and super abilities (coming soon!) and require distinct items in order to level up their skills.
Common: to upgrade its skills, you need bronze medals; up to level 10.
Rare: requires silver medals to upgrade; up to level 15.
Legendary: only gold medals will upgrade its skills; up to level 20.
You can win medals to level up your favorite champions through the Mystery Rolls (see more here) or by purchasing them in the shop.
When you reach the maximum level with a certain champion you will be awarded a bonus (Max Level Bonus). You also have a bonus for mastering a champion (Mastery Bonus).