Ada Gillyflower was a human living in Victorian Yorkshire. She was blind and had scars on her face from her mother, Winifred's, experiments with red leech poison to create an antidote from the toxins.
Ada attended her mother's sermons and was used as an example of human ignorance. Her mother claimed that her blindness was caused by her father during a drunken rage.
When the Doctor was restored, he told Ada the truth about her mother's actions towards her, and she turned against her mother, even beating her mother with her cane after her mother admitted the truth about her experiments. After Clara destroyed the rocket launcher, Ada's mother held a gun to Ada's head to hold the Doctor back. When her mother was killed by Strax, she destroyed Mr Sweet. After these fateful events, the Doctor asked her what she would do with her life, to which she replied, "There are many things a bright young woman can do". (TV: The Crimson Horror)
Due to her blindness, Ada was a very lonely, insecure girl who devoted herself to her mother. Her insecurity was shown especially when she locked the Doctor away after discovering he survived the poison, affectionately referring to him as 'her monster'. Despite this, she seemed confident in her abilities, calling herself 'a bright young woman' and that there were a lot of opportunities for her.
While seemingly unstable, Ada was certainly far more mentally stable than her mother, who used her as a guinea pig to create an antidote for Mister Sweet's poison, which caused her blindness. She was outraged when she discovered what her mother had done, calling her a 'perfidious hag' and believing all the years she had spent looking after her meant nothing to her. In her mother's final moments, Ada coldly stated that she would never forgive her, to which her mother expressed pride, saying "That's my girl". (TV: The Crimson Horror)