The reader learns several things about Mary Maloney in the early parts of the story. If you are referring to something very concrete about her that the reader learns, then I believe the most important detail is the fact that Mary is six months pregnant. It's a key detail to the story, because after she kills Patrick, Mary is perfectly willing to take the punishment. But she doesn't know what will happen to the baby, so she decides to cover up the killing for the sake of her unborn child.
As the wife of a detective, she knew what the punishment would be. It made no difference to her. In fact, it would be a relief. On the other hand, what about the baby? What were the laws about murderers with unborn children? Did they kill them both -- mother and child? Did they wait until the baby was born? What did they do? Mary Maloney didn't know and she wasn't prepared to take a chance.
The reader also learns that she is desperately in love with Patrick. She's practically a "Stepford Wife." Mary is the quintessential doting wife. She is sitting in the front room eagerly awaiting the arrival of her husband. He is the reason for her existence.
The room was warm, the curtains were closed, the two table lamps were lit. On the cupboard behind her there were two glasses and some drinks. Mary Maloney was waiting for her husband to come home from work. Now and again she glanced at the clock, but without anxiety: She merely wanted to satisfy herself that each minute that went by made it nearer the time when he would come home.