Sachie Murata is one of the sexual slaves that the master of the manor, Renji, has taken in to train as a slave. She is usually seen wearing the same uniform as most of the other maids and it seems her body has been altered to satisfy the strange whims of Renji, at one point having tubes implanted in the sides of her stomach to allow Renji to bloat her with water. Her backstory before the manor is not explored and yet, it seems she has not entirely been stamped out in terms of resitance.  In the first episode we see that she resists being tortured early on, which earns her a slightly dark treatment with candle wax and later on, when she consorts to talk with the main character, Yuu as well, she is punished by being made into a human toilet, which in itself, is horrible. She does, as the show progress, fall slightly for Yuu, finding her a beautiful being amid a sea of dark treatments and blatant abuse. They make love, somewhat, after Yuu is given a males member in a bizarre change to her body and yet, her final threat is the most dark point in the series at this point, her body being decapitated and her head being shown to the horrified Yuu.  In short, however, Sachie's small acts of defiance are welcome in the show and in the end, she accepts her fate rather then go kicking and screaming, as Renji seems to have intended.  (Sources: ACD)                 

Yuu is a girl who has lost her memories. She is mailed into a sadistic house of bondage where she and others are forced (or not) to serve their master, Renji, but there are serious consequences for those who fail.

Bonus episode for the hentai anime Bondage Game: Shinsou no Reijoutachi.