Warning: The following contains spoilers for The Night Beyond the Tricornered Window, Episode 7, “Atonement”, now streaming on Crunchyroll.
Hiura Erika has consistently been one of the more interesting characters in The Night Beyond the Tricornered Window. A kind of cross between Mikado and Hiyakawa, she can be ruthless when she needs to be, like when she cursed Hanzawa’s wife in Episode 6. But she is also tormented by her fear of the mysterious “sensei” and her own guilt, and she doesn’t know how to escape. In Episode 7, Erika gets the conviction she needs from Mikado and finally decides to change her ways, though the process is neither simple nor painless.
But like Hiyakawa, Erika is also naturally drawn to Mikado’s positive energy and even asked Mikado for help with homework in Episode 5. This shows that Erika trusts him enough to invite him into her world. Thus, when Erika sees Hanzawa, who she knows is investigating her, she feels betrayed by Mikado.
“Belief” is a powerful concept within Tricornered Window's world, and its power is demonstrated again when Erika loses her belief not only in Mikado, but also in her own fate. She feels immense self-hatred for what she's done and, knowing that the police could catch up to her at any time is the ultimate trigger that makes her lose control over her powers. According to Mikado, it looks like she poisoned herself.
Erika’s power is so strong that even Sakaki, who has no psychic abilities, is affected by her power and feels sick just by being near her. But surprisingly, Mikado feels nothing and can even dispel the negative aura around Sakaki. He definitely inherited his mother’s power to cleanse negativity, but until now, he's had no idea how to use it.
Mikado uses his power to help Erika and remembers the various psychics mentioning “belief.” He asks Erika to believe in him because he believes in her innate goodness and her will to live. Erika finally confesses that she doesn’t want to keep doing her work and wants to be a good person. Her own faith helps her rein in her powers and returns her back to normal.
This ordeal helps Erika take the first step towards change by atoning for what she's done. She starts by helping to lift the curse she placed on Hanzawa’s wife, Saeko. Even though Saeko says she doesn’t believe in the occult, she does believe that Erika is truly sorry, and this allows Erika’s powers to work smoothly to lift the curse.
Erika’s curse works by using the dead spirits inside her to torment others. Since she is a necromancer, the spirits of the dead all want to enter her body, but once she allows them inside, she experiences their death and all the agony and despair that comes with it. Her power is an extremely painful one and that's probably why she doesn’t enjoy using it all that much. Case in point: When she asks the spirit that's with Hanzawa’s wife to come back, she passes out.
Yet what finally moved Erika wasn't the pain of experiencing death, but the kindness of Hanzawa and his wife, even though they know what Erika did, they are nonetheless concerned for her wellbeing. Hanzawa simply wants her to understand the consequences of hurting others, and hopefully help her stay away from the path of evil.
This experience, along with Mikado’s empathy for her fears and loneliness are the things Erika needed all along to change her life. She needed to realize that she has a choice, and there are many people close to her who are willing to help. Even Sakaki, the man hired by the cult to watch her, actually genuinely cares about her and secretly wants to help her escape her situation.
Erika was in a similar situation as Hiyakawa -- they were both trapped by an organization that abuses their powers. But Hiyakawa escaped by inflicting pain and death on others while Erika did so by confronting them. Erika has found the saving grace she needs to change, but it seems that Hiyakawa still has ways to go on his own path towards redemption.
