JOKER is an enemy character introduced in Persona 2: Eternal Punishment, a deliberate callback to the character in Persona 2: Innocent Sin.
Following the events of Persona 2: Innocent Sin, the events are erased from everyone's minds.
While Tatsuya Sudou is committed to a sanitarium, Nyarlathotep influences him by informing him of the existence of the Other Side, and empowers him to become the next Joker (without outright restoring his memories of that dimension). Sudou then escapes as JOKER to murder people by spreading rumors, having killed the sanitarium's staff during the beginning of Eternal Punishment. JOKER can be reached by someone calling themselves on their cell phone. According to the rumor, he will kill anyone you ask.
Realizing how dangerous he is, Tatsuzou Sudou, Sudou's father, attempts to kill him using members of the Taiwanese Mafia. This fails and all of them are slaughtered. Sudou escapes from the sanitarium for good and lures Maya Amano and Jun Kashihara to the Sky Museum, which he proceeds to set on fire. Sudou is still loyal to the original Joker, so he attempts to get Jun to remember the events of Innocent Sin. This fails, and Jun pushes Sudou into the flaming building's collapsed roof. Sudou emerges alive, but scarred and with only one eye. He is killed by Tatsuya Suou.