Rank is represented by the number of stars displayed on a hero's portrait. Generally speaking, higher-ranked heroes have better stats, better stat growth, and more useful skills. The highest rank a hero can currently achieve is Rank 6.

Rank 1
Rank 2
Rank 3
Rank 4
Rank 5
Rank 6
Star silverStar silverStar silverStar silverStar silverStar silver Star silverStar silverStar silverStar silverStar goldStar goldStar gold

Both main and support heroes increase their ranks through fusion. Support heroes can increase their rank most efficiently through fusing multiples of the same cards to itself (eg. fusing Dober to another Dober). However, because multiples of the main hero card aren't distributed to a single account and are untradable, the only way to promote a main hero is by fusing other heroes to it (eg. fusing Dober to the Sniper). The same method may also be applied to support hero cards, but it is much slower than fusing the same cards to it.

For more information regarding increasing a hero's rank (promotion), see here.

Heroes of different seasons have different stats and growth, thus rank should only be used as a very general point of comparison within the same season and between heroes with similar traits. Comparing heroes between seasons may prove to be an inaccurate measure (eg. Season 2's 3-star Vera has higher STR and VIT than Season 1's 5-star Karinna, so it would be inaccurate to say that Karinna is tankier simply because she has a higher rank), and in such cases, base stats and skill traits should be compared instead.