Motoko Eguchi is Takeshi Ōmura's cousin, living with him and her mother, Sanae Ōmura, following the destruction of his house by fire. She displays a tomboyish personality and frequently teases Takeshi, often incorporating erotic magazines into her mockery. Her provocative behavior includes wearing revealing clothing around the house and making sexual advances toward relatives; the extent of these actions beyond harassment remains unclear. She exhibits a complex dynamic with Takeshi, showing a non-serious attraction that coexists with underlying familial affection.

Upon the angel Elle's arrival in the household, Motoko immediately develops a liking for her. Sanae routinely employs physical discipline to curb Motoko's excessive conduct, demonstrating a pattern where her actions require external restraint. Her nickname "Ero-neesan" originates from her unconventional preferences and shameless demeanor, particularly her interest in adult magazines targeted at men.

Physically, Motoko possesses auburn hair and brown eyes. Her character portrayal, backstory, and narrative role remain consistent across official manga volumes, the original video animation, and other media.