Mrs. Reynolds is the housekeeper of Pemberley. It is implied that she took care of both Fitzwilliam Darcy (now her master) and his sister, Georgiana, when they were younger. Mrs. Reynolds received Elizabeth and her aunt and uncle when they visited Pemberley. She spoke about Wickham and more at-length about Darcy and Georgiana; it is her account that furthered Lizzy's reconsideration of Darcy's character.
There is little known about Mrs. Reynolds' life outside of her life as the housekeeper at Pemberley, a job that she apparently took on when Darcy was four years old.
"... I am afraid he has turned out very wild."
―Mrs. Reynolds on WickhamMrs. Reynolds knows Wickham because he is the son of her "late master's", or Darcy's father's, steward, and was brought up by Darcy's father "at his own expense." Since she was, presumably, the housekeeper throughout Wickham's stay at Pemberley, it can be inferred-- on Lizzy's part as well-- that she knows Wickham's character rather well and that her negative description of him was accurate.
The Darcy Family Edit
"I have never known a cross word from him in my life, and I have known him since he was four years old."
―Mrs. Reynolds on her masterMrs. Reynolds helped to raise her now-master Darcy, and obviously holds him in high esteem. When speaking with Elizabeth and the Gardiners, she was vocal in her praise of him. She recounted his younger self to Lizzy as already extraordinarily well-tempered and generous, and opined that he had kept these traits into his adult years. She also recounted his deeds of goodwill and described him, in many words, as good-tempered, unselfish, honorable, and kind-- and, as Elizabeth in-part concluded, a good brother. Overall, her favorable portrayal of Darcy caused Lizzy to further question and reconsider her previous views on him, owing that Mrs. Reynolds' account was "the praise of an intelligent servant" in her master's absence and she could just as well have spoken negatively of Darcy without fearing repercussion.
"Oh! yes-- the handsomest young lady that ever was seen; and so accomplished!"
―Mrs. Reynolds on GeorgianaMrs. Reynolds is fond of Georgiana as well and speaks of her talents enthusiastically. She also revealed Darcy's fondness for his sister and how considerate he is of her happiness in her conversation with Lizzy and the Gardiners, in a way countering Wickham's claims.