The character known as "Younger Sister" appears solely in Operation 02: Sacred Girl. She lives a reclusive existence within a mansion alongside her older brother. Her brother hosts nightly parties centered around sex, observable on his surveillance system. Her background depicts a sheltered life within this isolated mansion setting, with minimal interaction beyond the household.

Her brother's obsession leads to a consensual sexual encounter between them, marking a shift in their relationship dynamic. This development occurs without coercion within her narrative arc for the episode. She does not appear or receive development in other episodes, seasons, movies, OVAs, or spin-offs related to the series.