Eri was abandoned on the steps of a church in Hiroshima, Japan and subsequently raised there until she was old enough to escape. After leaving the Church she became the leader of a street gang, in which her actions had later brought her the attention of the Intelligence Service; they hired her as an assassin and spy for the government. Though she excelled at her new role, the missions began to weigh heavily on her conscience and she asked to be transferred to S.P.A.R.R.O.W.S, a request that was granted in light of her exceptional performance record.
Starting from a rank of sergeant 2nd class in S.P.A.R.R.O.W.S., she too like those she worked with have climbed up the ladders of ranks. Although her partner Fio is of a superior position, Eri has been known to show a sort of parental love to her officer from her experience of leadership in the past life.