Erika Sako is the older sister of Six, who died at an early age by falling down some stairs. After Six's soul is trapped inside Paradigm X's VR Haunted Mansion, a rescue attempt is made by the protagonist and Nemissa. As they travel through the Haunted Mansion they hear voices of a little brother playing with his older sister.
While they attempt to free Six, Erika hinders their path, teleporting them to another section in the maze three times over, until she reveals that she is Shingo's older sister, and being upset that he forgot about her, she's finally glad that they'll be together forever now. Before this conversation, it's revealed that Six accidentally pushed Erika down some stairs, resulting in her death. The player and Nemissa later defeat Erika and her minions -- Goggie, Lukoje, and Bugs -- resulting in Six being freed from the simulation. Whether each minion is summoned in battle or not depends on whether you choose to take a younger Six with you or to leave him alone in the flashbacks, with them appearing if you do.