|Japanese Name||間宮 えりか|
- Age: Unknown, most likely 15
- Eye colour: Pink
- Hair colour: Blonde
- Japanese pronoun: Uses atashi (あたし) when speaking to Kirika; watashi (わたし) when speaking to her step-father.
- Known relatives: Her mother, father, and step-father Mamiya.
- School: Unknown. She is said to be in middle school.
Erika and Kirika were best friends since childhood, and promised to stay friends even as adults. When Erika's parents announced they were getting a divorce and Erika would be forced to transfer schools, she got scared. In her fear, she shoplifted from a bookstore, running away when she ran into Kirika and dropping the book she'd stolen. Kirika was blamed for the shoplifting, and Erika moved away before they could speak again. Kirika believed Erika betrayed her and they drifted apart.
Erika during Extra Story
Erika appears in Noisy Citrine as a prominent character, and has a brief appearance in The Last Agate.
During Noisy Citrine, Erika spots Kirika from the train. They meet face to face when Kirika bumps into her by accident a little while after. Upon remembering how she was betrayed by Erika in the past, which caused their friendship to crack, Kirika pushes her and runs away.
The two girls meet later, but Erika was depressed due to her newly married mother and new stepfather. She becomes angry when her mother calls her useless for not calling her stepfather "Dad", which makes her vulnerable to a witch. She gets a witch's kiss on her neck, causing her to wander to the witch's barrier in an abandoned paint factory, and prepares to hang herself at the lowest point of the barrier. Kirika, who recently contracted with Kyubey and became a magical girl, arrived to rescue her. Erika tells Kirika that she wanted to disappear because of her uselessness, and asks if Kirika's hates her for what happened at the bookstore. Erika then jumps out of Kirika's arms, intending to die. Kirika catches her and replies that they are different now, and they will continue to change. She kills the witch, which prompts Erika to ask what Kirika is. Kirika replies that she's, "nothing special, just a magical girl." After defeating the witch, Kirika decides to forgive Erika.
They continue to keep in contact through letters even after Kirika became close to Oriko. The letter Kirika reads out loud says that Erika has begun getting along with her parents.
During The Last Agate Erika is seen with her mother and father, packing supplies for a trip to her grandmother's house during the supercell (Walpurgisnacht) event.
- Mamiya is the family name of her step-father. It's not clear if Mamiya is actually her current family name or not, but she has been given no other surname.