The information below derives from OVER HEAVEN which was not written by Araki. As such, it may not be considered canon.
Dio Brando's mother is a noble, proud, pure, and righteous person. She expresses kindness to almost everyone around her. Despite her dire circumstances, she often goes out of her way to help those in need, often to the her own detriment. She is principled; above all else she values doing noble and selfless deeds, as she believes that such way of living leads to heaven. This idea is central to her worldview, seemingly influencing most of her decisions.
She doesn't tolerate violence in any form; when Dio tried to fight back against people making fun of his mother, she told him that he won't go to heaven relying on violence.
Although her strong convictions and selfless actions, which to many seem foolish, only earn her mockery and contempt from those around her, she never harbors any ill will towards them. When the aggression is coming from her own family, she makes every attempt to steer them in the right direction. She deeply loves her family, in spite of the scorn and abuse she receives from them. She believes that Dario is a good man who just needs to stop drinking.
Coming from a wealthy family, Dio's mother is well educated.[3][4]
Concludes non-canon section.
OVER HEAVEN
The information below derives from a source not written by Araki. As such, it may or may not be considered canon.
Dio's mother seemingly comes from a wealthy family. Later in life, she moves to a poor town and marries Dario Brando.
Although she and her family often suffer from starvation, she regularly gives away the little money or food she has to those in need, such as children, the elderly, and sometimes animals. These acts of charity unfortunately only result in mockery, as the residents of the town consider such actions to be foolish. Despite agreeing with them, Dio Brando, being her son, cannot stand people making fun of his mother; but whenever he would fight back, she would scold him, saying that he won't go to heaven relying on violence.
To Dio, she appears to be obsessed with the concept of heaven, as she takes every opportunity to remind him of it, which earned her a great deal of contempt from her son. Whenever Dario, in his drunken frenzy, beats her, Dio sees it as a justifiable punishment for her foolishness. Knowing that attempting to escape or minimize the abuse is futile, she always tries to convince him to stop drinking. Once, as a result of the beatings, she miscarries.[3]
Sometimes she would sing a lullaby to Dio before sleep. The lullaby contains 14 phrases: Spiral staircase, rhinoceros beetle, Desolation Row, fig tart, rhinoceros beetle, Via Dolorosa, rhinoceros beetle, singularity point, Giotto, angel, hydrangea, rhinoceros beetle, singularity point, secret emperor. Many years later, Dio would use these phrases in his plan to achieve heaven.[6]
At one point, Dio tells his mother that her way of life is impractical and that heaven does not exist. She, looking somewhat upset, explains to him that he'll understand it when he gets older, claiming that heaven exists and there is a way to get there, so one must live for the purpose of doing so.
Dio's mother dies abruptly. She is buried in a crude funeral by Dario and Dio.[4]