Sugihashi Junko

She is a wife who lives in building B with her husband,

She is a cool person and had a history of refusing of going to school when she was a student.

She used to date her teacher and rumors started circulating which caused her to avoid going to school.

Spoiler

she sleeps with her father in law not because she wants to but for a few reasons. Her husband is impotent and her father in law is someone she knew in the past. Tooru helps put a stop to this with her.