Injustice 2's achievement list has seemingly revealed the identity of one of the game's unannounced characters (via XboxAchievements).
Two achievements (I Love You, I Hate You; and Why Aren't You Laughing?) specifically reference Joker who hasn't been announced by NetherRealm and Warner Bros., as of yet.
While Batman's nemesis is in Injustice: Gods Among Us, he hasn't been confirmed as a playable character for the sequel as of yet, but the achievements list confirms the world's second most frightening clown will be returning to the series.
Pennywise is obviously the scariest.