You are not given Joker when you log in, unlike when Marvel’s Avengers dropped a new character. Instead, you are told to grind to Season Rank 35 to fight Braniac to then unlock him. This is not battle pass rank or mastery rank, but a separate, third rank that will force you to grind somewhere between 10-20 missions, depending on the mission, to reach that level. This is old content with your old characters, but your difficulty is capped on things like Incursions making them impossibly easy with your already-powerful builds.
I thought the point of the grind was you could level up your new character. Having to grind to unlock a secret third rank doesn’t sound fun.