Reverse Flash
There are a couple of ways to unlock this evil speedster. The first is to have bought the Ultimate Edition of the game ($100). This grants immediate access to the Reverse Flash premium skin in Injustice 2, changing the look, voice, and dialogue of the character. If you didn’t buy the more expensive version of the game, you can simply save up your Source Crystals (6,000 of them) and purchase it from Flash’s loadout menu.
Jay Garrick Flash
This version of the Flash isn’t obtainable through any of the editions of Injustice 2 but can be unlocked in game by paying Source Crystals or earning it as a reward.
John Stewart
Considered by some to be the best Green Lantern, you can put on the summoned tights and stoic attitude of John Stewart as you take the fight to your opponents. You can obtain this skin (which is complete with unique voice and dialogue) by purchasing the Ultimate Edition of Injustice 2 or by paying up 6,000 of your precious Source Crystals.
Power Girl
You have a few ways to unlock this Earth-Two variant of Supergirl. She comes in the Digital Deluxe and Ultimate Editions of Injustice 2 (they cost $80 and $100 respectively) already unlocked and ready to rumble. You just have to equip her as a skin to Supergirl. And, like all the other alternate characters you can unlock, she has her own dialogue and look. If you bought only the standard edition you’re still in luck. You can buy this skin for 6,000 Source Crystals.
That’s all you need to know to unlock all characters in Injustice 2. For more on Injustice 2, be sure to check out our growing wiki.