Madame de Saint-Ange is the only character who is present for the entirety of The Bedroom Philosophers. The boudoir in question is hers, and she has taken the innocent Eugenie under her libertine wing at the request of Eugenie's father, one of Madame's lovers.
Madame de Saint-Ange is married to an older gentlemen who allows her to take lovers as often as she like so long as she regularly satisfies is fecal-sexual proclivities. She also regularly engages in sexual congress with her brother the Chevalier de Mirvel. It is the Chevalier who arranges for Dolmance's involvement in Eugenie's education.