In both realities, his personality is the same despite taking different roles; being insane, quick to anger, apathetic, cruel and has no qualms of sacrificing anyone, including his subordinates to achieve his goals. He is also quite delusional and unstable due to the visions his voices had him see, making him an ideal candidate to spread destructive rumors. In fact, his insanity results in him willing to commit any degree of crime in order to take revenge on anyone who stands in his way including his father and Tatsuya Suou, and is thus highly dangerous and impossible to reason with.
Tatsuya Sudou was a senior at Seven Sisters High School when Akinari Kashihara was teaching history there. He was a nervous, twitchy boy who was terrified of his cruel father. He also heard "voices" in his head, which made him even more nervous. He developed a deep affection towards Kashihara, who acted as a substitute father in the place of Tatsuzou Sudou because the latter only regarded him as a disgrace; Kashihara thought that Sudou's voices were direct messages from the Maiyan aliens and wrote them all down into the book In Lak'ech. His descent into outright madness was compounded partly because of the vicious rumors other students would spread after him, making him even more unstable and volatile than before.
Shortly afterwards, Sudou set fire to the Alaya Shrine only for Maya Amano to break out of the shrine using her Persona, Maia. Dubbing her a "witch", Sudou attempted to kill her, only to flee after Tatsuya Suou burned out his eye using his Persona. As explained in Eternal Punishment, Suou's father was investigating Tatsuya Sudou's involvement of the arson but was halted and forced to resign dishonorably due to Tatsuzou Sudou's interference. After being released from the Morimoto Sanitarium, Sudou began stalking Maya and threatening her. He eventually joined the Masked Circle as King Leo and served as the right hand man to Joker. Ixquic acts as his subordinate, although he has no qualms of trying to sacrifice her when she questions him. Because Joker is in fact Kashihara's son, his loyalty towards Joker possibly stems from the physical resemblance between the two. He uses the Persona Reverse Vulcanus during battle.
Tatsuya Sudou in Eternal Punishment is liberated by Nyarlathotep after being confined to the Morimoto Sanitarium by his father, Tatsuzou Sudou, for committing serial arson-murder 10 years ago, which was investigated by the father of both Tatsuya and Katsuya Suou, but the investigation was abruptly halted by Tatsuzou. Nyarlathotep informs him of the existence of the Other Side and gives him the opportunity to escape by manipulating rumors and becoming the JOKER. (As explained by Tatsuya Suou, Sudou has not "restored" any memory of the Other Side.) He becomes a contract killer, whom the caller summons by dialing their own cellphone number. He wears a paper bag resembling the Other Side's Joker mask.
JOKER is introduced in the game when he kills Principal Hanya and later confronts Maya Amano himself, claiming to have been "ordered" to kill her. Instead, he attempts to get Maya to remember the Other Side, but fails and summons demons to kill her and her party. They later catch up to him at the clock tower, where he is attempting to get Anna Yoshizaka to remember her role as Lady Scorpio, one of the executives of the Masked Circle on the Other Side. He puts Maya's party to sleep and attempts to finish them off, but is stopped by Tatsuya Suou.
With the aid of Baofu, Maya's party heads to the Morimoto Sanitarium where Sudou is being held, only to find that he has broken free and has killed the assassins sent by his father to kill him. Maya later confronts Tatsuya Sudou in the Director's Office, where he reveals that he got his power by spreading rumors and that the "voices" told him everything. Sudou then picks up the phone of one of the dead assassins and tells the person on the other line to tell his father that he is coming. He then summons a Hellhound and leaves, instructing them to go to the Sky Museum if they survive.
Once Maya's party reaches the Sky Museum, they learn that Sudou had lured Jun Kashihara to the museum. Sudou then reveals over the intercom that he has set bombs to go off in 30 minutes, "just like the Other Side". Maya manages to save the children there and reach the top of the Museum. Once there, she finds that Jun has been taken hostage by Sudou, who attempts in vain to get Jun to remember his role as the Joker. Tatsuya Suou attempts to save Jun and the remaining bombs go off, creating a hole in the roof which Jun pushes Sudou through. Maya and the others attempt to escape via blimp, but Sudou, clearly enraged at having lost his eye again, manages to sneak on board and attacks them, but is killed by Tatsuya Suou.
Tatsuya Sudou's clothes bear some resemblance to The Reaper's garments, from Persona 3 and onwards. The Reaper is also missing the same eye that Sudou is missing, lending to the resemblance.
Scenario writer Tadashi Satomi expressed his grievance at the Japan's law which remits convicted murderers who are diagnosed with mental disorder (Tatsuya Sudou) in the additional scenario.
