Who was the mother of Fabrizio?

Fabrizio's mother is the Marquise del Dongo. On page 10, chapter 1, the French soldier Lieutenant Robert describes her in glowing terms.

Lovely, angelically soft eyes and that pretty dark blond hair that so perfectly framed her charming face. There was a painting of Herodias by Leonardo in my room: you'd have thought it was her portrait.

In comparison, he claims her husband the Marquis del Dongo is a coward. When the French entered Italy, and later Austria, he left his wife to deal with everything while he took refuge in his castle.
She dealt with it well and became thought of as a good person. Though her husband didn't give her much money, she helped out her poor sister by secretly giving her her pearls. Equally, Lieutenant Robert describes how warmly she welcomed him and how actively and emphatically she listened to him, even though he claimed he looked a disgrace.
The author states the Marquise "adored" her son. When Fabrizio boards at a Jesuit college in Milan, she borrows money from her sister-in-law to visit him. She is upset when he decides to leave the country, but does all she can to help him.

The Marquise burst into tears on hearing of her son's strange plan... When she became convinced that nothing in the world, other than the walls of a prison, could prevent his departure, she gave him what little money she owned, then recollected that she had in her possession since the previous day, eight or ten little diamonds, worth perhaps 10,000 lire.

Marquise Del Dongo was Fabrizio's mother. Lieutenant Robert described her as a woman of much beauty, with auburn hare, eyes of "angelic sweetness" and an oval face. He also said that Leonardo Da Vinci himself could of painted her, for she was that pretty.
She is a truly warm hearted woman. She lives in a lovely home with marble in the dining room and servants galore. When married, she brought her husband a dowry of 800,000 francs. Her husband however is greedy and cold hearted, only letting her have 80 francs a month, even though she brought him so much. She is described as timid, yet she does manage to sneak diamonds from her jewelry as a way to help out his sister, who he had abandoned for going against his will. It is seen that even though his sister is starving in Paris, he refuses to help her, which is why in the end Marquise Del Dongo helps her like that.
Her character is a total contrast from her husband, as she is sweet where he is mean. She is warm where he is cold, and she is compassionate where he has no heart.

Fabrizio's mother was the Marquise del Dongo. She is described by Lieutenant Robert as a beautiful woman in full bloom with auburn hair, an oval face, and eyes of "angelic sweetness." According to Robert, she is so lovely she could have been painted by Leonardo da Vinci.
She lives in a splendid home with a marble-paneled dining room and many servants. She is a kind-hearted woman who brought her husband a dowry of 800,000 francs when they married. Unfortunately, her husband is a greedy and hard-hearted man who only allows her 80 francs a month in allowance. Although described as very timid, she manages to take a few diamonds from her jewels, which her husband keeps carefully locked up. She sends these to his sister, who married against his will and who, because she defied him, he will not help out, even though she is starving in Paris. The Marquise is depicted as a gentle woman of goodness and compassion, a sharp contrast to her sordid husband.