I have seen several mentions of Galdós likely basing a few characters on his mother, but this footnote (on page 78 of Angel Guerra) provides a good summary:
The portrait of Doña Sales is almost ertainly based on his mother, referred to by all the family as Mama Dolores. She was a loving but very domineering mother—so much so that the author is almost always known by Galdós (Dolores’ maiden name) rather than Pérez (his father’s name) or Pérez Galdós, as would be correct. Benito was the youngest of ten children, and only managed to give her the slip when she sent him off to study at the University in Madrid in order to separate him from what she considered to be a budding undesirable connection with one Sisita, the illegitimate daughter of an uncle. Once away, he steered fairly clear of the Canaries—and of her. He was close to his family and certainly loved her, but he seems to have felt it wiser to love her from a good safe distance. There is an earlier and far more horrifying depiction of her in the protagonist of Doña Perfecta (1876). The portrait in Angel Guerra is gentler, more disposed to give her credit for her strengths as well as her flaws.