Sister (also known as Sis or Onee) is a 32-year-old member of the Eden of the East group, which operates an online image recognition and recycling system. Though not officially enrolled as a student, she has taken classes across multiple university departments and oversees the group's activities. She functions as an assistant to Eden's leader, Kazuomi Hirasawa, frequently encouraging and coaxing him in his responsibilities.

Her personality includes a noted fondness for "cute things" and distinct erotic interests, which unsettle her NEET companions within the group. This aspect is presented neutrally as part of her interactions. She maintains an enthusiastic demeanor and actively participates in Eden's technological initiatives and strategic operations.

During the events of "Paradise Lost," she engages directly in the conflict against Seleção No. 1, Daiju Mononobe. She assists in hijacking the trailer containing Mononobe's Juiz supercomputer unit, a critical action to prevent him from destroying the remaining Seleção support systems as part of Eden's efforts to protect their operations and aid Akira Takizawa.

Her role remains consistent as a supporting member of Eden across the series and films, with no significant character development, changes to her background, or additional personal history or evolution detailed in the available materials.