The Other Side was a major plot point in the Persona 2 duology.
At the end of Persona 2: Innocent Sin, the group of Persona Users spearheaded by Tatsuya Suou destroyed Nyarlathotep. However, the deranged teacher Maya Okamura used the Spear of Destiny to kill Maya Amano, fulfilling the penultimate line of the Oracle of Maia. Her friends, desperate to ensure her survival despite the Spear's legendary powers, agreed to bargain with Philemon to destroy the timeline in which the events had come to pass. However, this would require them to embrace the new reality, one in which they had never met.
All of them accepted very reluctantly, except Tatsuya himself, who refused to have his mind wiped of his memories and was forced into the new timeline still holding the memories of the earlier timeline, now dubbed the Other Side. Because of this, he retained his Persona-related abilities and much of the knowledge he had accrued on Sumaru's powers and Nyarlathotep's role in the disasters plaguing it.
Nyarlathotep's plans in Persona 2: Eternal Punishment centers primarily around the Other Side, a now-isolated timeline in which his plans to ensure the death of reality had succeeded, and Tatsuya, the key to absolute destruction (as he held the necessary memories to force the restoration of the Other Side). Other key players in his master plan were the Sudous - Tatsuzou Sudou and Tatsuya Sudou, who were used as peons in their master's ploy to force an overlap between the two realities.
Tatsuya tried to confront Nyarlathotep by himself, but was defeated and branded with an emblem to remind him of the fact that if any of his friends (Eikichi, Lisa, Maya, or Jun) ever recovered their memories of the Other Side, Nyarlathotep would be able to bring back the dead reality.
Depending on choices made at the Monado Mandala, Eikichi and Lisa could indeed recover their memories, though Nyarlathotep is unable to recreate the Other Side, as the game ensures that Jun does not recover his memories. At the second defeat of Nyarlathotep, weary and tired, Tatsuya painfully agreed to forget to ensure the Other Side would remain inaccessible (though if anyone else had their memories awakened, they would still remember). As he agreed, he faded back into the Other Side, leaving behind the Tatsuya native to the new reality and saying that he would help rebuild what is left on the Other Side.
Changes in the Timelines
- Akinari Kashihara never died and instead he and his wife and son found a measure of tranquility and happiness, ensuring the Masked Circle never was born.
- Tatsuya Suou, Maya Amano, Lisa Silverman, Eikichi Mishina and Jun Kurosu never met.
- Jun retained the Kashihara surname.
- Tatsuya Sudou's crimes changed from arson to murder (the Araya Shrine was never burned down; instead, Shiori Miyashiro's little brother was killed by Sudou).
- All the deceased during the Innocent Sin timeline (Other Side) were revived and restored with no memory of the events.
- In Lak'ech was never written, erasing the existence of the Fuhrer and his Last Battalion and ensuring that the Oracle of Maia would remain unfulfilled.
- MUSES was formed with the full cooperation and understanding of all three members.
- Several shops moved to different areas of the city, i.e. the Kuzonoha Detective Agency is now located in Aoba. Eikichi's father states that he moved the location of Gatten Sushi because of a Wang Long fortune.