A young girl who was left to die alone in the old school building of Seikyou Private Academy long ago. She has no recollection of how she died and is currently searching for clues together with Teiichi Niiya. As a ghost, she is normally invisible to the people in the school, with the exceptions of Teiichi and Kirie Kanoe, a blood relative who can also see her.

Apparently, she has grown fond of Teiichi and has expressed her desire to be by his side forever.

Except for Teiichi, the other members are unaware that Yuuko is actually the 'absentee' president of the Paranormal Investigations Club.

In her first appearance, she warns Niiya about Yuuko.

Teiichi is a pupil in the school Yuuko died about 50 years ago, he once heard rumors about a spirit in the school who was left to die alone. Teiichi is able to see her and wants to help Yuuko to find out why she died.

The most enthusiastic member of the Paranormal Investigations Club, Momoe is a year older than Niiya and begins to develop feelings for him after seeing his dedication to solving the Seven Mysteries. Like most people, she is not able to see Yuuko. Momoe also becomes close friends with Kirie by the time of the school festival. Officially, Momoe is a member of the school's Third Newspaper Club, but she is able to participate in the activities of both clubs due to the unofficial nature of the Paranormal Investigations Club.