Erii is a young Mushitsuki who controls a pack of White Swallowtail butterflies to manipulate electrics. She can gain access to all equipment that requires electrics to operate, and can even have visions by sending currents through any medium that conducts electrics. These proven to be very powerful abilities in the urban environment. She can also summon her White Swallowtail butterfly to shoot electric beams to paralyze opponents.
Her dream is to "know every secrets about the world." In Mushi-uta Bug Vol. 5, ch.18, She was discovered by Central SEPB, and Daisuke brought Arisu and Neiko to investigate. Arisu used her ability to "challenge" Daisuke and his company but Daisuke managed to break every obstacles. When she was discovered, her brain was about to burnout by uncontrollably accessing too much information, but she was too afraid to shut down her power. Daisuke did an unusual act, which was to spare her, and convinced her to shut down power with Arisu.
Erii was later sent to Eastern Central SEPB under the supervisor of Inuko. She slowly learned how to control her power and became a high ranked member in the intelligence section of Central SEPB.
Erii also made an appearance in the anime adaptation:
She was sent with Kasuou and Kirikiri to investigate Azami's power. She was left outside when Kasuou and Kirikiri got sucked into Azami's handycam. She also helped Daisuke to get into the handycam and saved him from it.
Erii holds a complicated love and admiration toward Daisuke, she also thinks that Daisuke and Arisu have the ability to fix the distorted world full of Mushitsuki. She lost half of her hope when Arisu sacrificed herself in the battle of "meteor night," so she put all her remaining hope on Daisuke and gradually became heavily depend on him. She showed an unusual anger to Daisuke when he decided to "betray" SEPB in Vol.3 and "reject" her. She also executed a failed assassination targeting Shiika because she thinks Shiika would "drag Daisuke away from his goal."
Deep in her heart, Erii never wished to stop the war between Mushitsuki. Yaeko used her thought to convince her to become a test subject by implanting Shinbu's body fragments which will grant her power to gain "many friends." Erii hated herself and felt she was betraying Daisuke, Arisu and all the Mushitsuki who battles hard to end the war. She wish Daisuke can stop and kill her. She gradually lost her personality and couldn't distinguish herself from Shinbu, the new creation of the mixture became the foundation of "C."
In vol.13, in order to escape C, Erii excluded her personality and transmitted it as digital information to a disposed satellite debris on earth orbit. To prove she have bravely fought to end the unending Mushitsuki civil war, so she could face Daisuke and Arisu with no shame, she planned to control the debris to strike earth to kill "Alpha," which will produce new Mushitsuki endlessly. Shiika and Neo-Mushibane tried to stop the debris and take her memories back. However, such plan was used by C trying to absorb the last bit of Erii to become even more stronger. Shiika risked her life to save Erii being absorbed, but was unable to save Erii's memory device. Erii changed the course at the last moment, with the help of Sanbikimei in Chiharu, left everyone no harm when impacted. Erii left her dying message to everyone: "please protect Yuu (from being taken by Oogui)."