Here's how the Joker returned for Injustice 2 despite his literal heartbreaking death in the original game. The Joker is arguably the most famous comic book villain of all time and steadily evolved from his original appearance as a comical prankster to becoming Batman's ultimate nemesis. His true name and backstory remain a mystery and he exists seemingly to create total chaos for his own amusement.
There have been a number of famous portrayals of the Clown Prince of Crime over the years, from Mark Hamill's voice-over work in Batman: The Animated series, Jack Nicholson in Tim Burton's Batman and Heath Ledger's Oscar-winning turn in The Dark Knight. Joaquin Phoenix will be adding to that legacy with 2019's Joker.
The Joker also played a pivotal role in 2013 fighting game Injustice: Gods Among Us. The premise behind the game sees Superman turn evil after the Joker tricks him into killing Lois Lane and setting off a nuclear device that wipes out Metropolis. Superman later interrupts an interrogation of the Joker by Batman and after the supervillain taunts him some more, the Man Of Steel punches a hole through his chest. This action leads to Superman setting up a tyrannical world order and the other members of the Justice League having to band together to stop him.
Fans might be wondering how the very, very dead Joker could thus return for 2017's Injustice 2. The sequel again finds Batman and his team attempting to restore some kind of order after defeating Superman and having to face a new supervillain group. The Injustice 2 Joker is revealed to be an hallucination brought on when the Scarecrow uses his fear toxin on Harley Quinn. During this hallucination, Joker appears and tries to sweet talk Harley into abandoning her new superhero persona and embrace her past, but she finds the strength to fight him instead and break free of the vision - and Joker's legacy.
This also explains why the Injustice 2 Joker is a little hunkier than usual. While some fans were disappointed the villain was only brought back for one scene in the game's story mode, it's a still a fun cameo and a good moment of character building for Harley. The hallucination aspect also recalls how the character was used in Batman: Arkham Knight and plans for a Jack Nicholson cameo in Joel Schumacher's unmade Batman Unchained.
Given some fan disappointment over the Injustice 2 Joker feeling somewhat forced into the story, the inevitable Injustice 3 will probably find a way to keep him around in a more concrete way. Injustice 2 also implies that Nekron - the embodiment of Death - is somehow responsible for bringing the Joker back too, so that's one possible way his return could be justified.