Premier Skins[]
In Injustice 2, the addition of Premiere Skins replace what would normally be unlockable characters (excluding Braniac - who is unlocked by beating the story).
These character skins are more than just gear - as they are a a full suit of gear that alters the character into a completely different hero or villain that shares that character's base moves. For instance, those who enjoy playing Supergirl can an unlock a premire skin for Power Girl - who looks different, but fights the same as Supergirl.
How to Unlock Premiere Skins[]
Premiere Skins are hard to come by, as currently there are only seven - and only a few can be unlocked by multiple means. All Premiere Skins can be purchased using Iso-8 Crystals.
Iso-8 Crystals are a special type of currency unrelated to credits, and are obtained much more rarely. You will also need a total of 6,000 Iso-8 Crystals to unlock each skin - but note that Iso-8 Crystals can be used for other things - like leveling up a character to rank 20 once you've done it for one character on your own.
There are a few Premiere Skins that can also be unlocked through other methods - which include DLC, linking your game to the mobile version, or owning the Ultimate Edition of Avengers Infinity.