The Joker was the person who started this entire mess with heroes fighting heroes in the original game. While that version of him died, there's a lot more Jokers out there via the multi-verse.
Injustice 2 looks like it's introducing a different kind of Joker, one that I'm not too keen on. He seems like he has some Jared Leto in him and his look just doesn't jive with me. As far as gameplay goes, he relies on his knife and various grenades.
He sounds about right, but I like I said, I don't know about the look.