Both Arika's and Nina's roommate. During the series's first episodes, Erstin mostly served as a bridge between the clumsy and spontaneous Arika, and the introverted and obstinate Nina, thus the trio became close friends, but she hides a dark secret that can shatter this friendship. Erstin harbors romantic feelings for Nina which was most evident in episode 6. During a pool scene, a creature was swimming around tickling the girls. Erstin thought it was Nina who was behind her and told her that if it's her, she's fine with it. Erstin quickly acquainted herself with Arika Yumemiya and didn't hesitate to help her in the best ways possible. She comforted Arika who at the time was being plagued by thoughts about her love towards Sergay. Erstin herself knew she wasn't as strong as Nina or Arika, so she did her best to support them in the best ways possible.

During episode 16, Erstin found out that Tomoe Marguerite was responsible for the many strange accidents that Arika was suffering from. When Tomoe was about to drop acid on her, Erstin shoved Arika out of the way and took the shot and fortunately suffered minor injuries. Strangely enough, she did not speak out.