Mary's mother is the main antagonist in the novel The Diamond Girls. She is an unnamed, mentally sick mom of a six-year-old girl Mary.
When Mary is born, her mother gets a post-natal depression. This leads her to abuse Mary constantly. She is obsessed with cleanliness and always wants her daughter to be a good, nice and lovable child. However, she has very strange ways to make it, due her PND.
She often tells her daughter she is a bad girl and sends her to bed early without reason. Once Mary comes with dirty or even little bit creased clothes, she is beaten. Her mother once grabs her and cuts her nails painfully, because they were "dirty".
Once Mary meets Dixie, an older girl from neighborhood, she tells her how nice her mother is. Dixie, believing it is true, later comes with her sister Jude to ask Mary's mother for candles. Although she is very kind at first sight, Dixie soon realizes there is only coldness and cruelty under her sweet mask, as she hears the mother slapping Mary for befriending "dirty and poor" Dixie.
After several days, Dixie wants to give Mary some of her dolls secretly, but is interrupted by Mary's father who invites her for dinner. The surprised mother isn't very happy to see the girl and even doesn't try to hide it. When Dixie and Mary's father later leave the room, the mother holds Mary's nose until she opens her mouth and then stuff crusts down her throat, forcing her to eat them, although she knows Mary doesn't like them. When Mary's mother looks up and notices Dixie who watches the scene in horror, she simply smiles and calmly tells her that Mary finally ate all crusts.
Dixie decides to stop Mary's mother, but has no idea how to do that. The only possibility is to tell Mary's father who, however, is always working. Luckily, the problem then solves itself when Mary jumps out of the window and falls on Dixie, breaking her legs. After that, all abusing is revealed and Mary's mother ends up in the psychiatric ward in hospital.