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 lied 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. She screamed at her and even beat 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 for all the suffering he caused. After these events, the Doctor asked her what she would do with her life, to which she confidently replied, "There are many things a bright young lady can do". She also said that it was time she "stepped out of the darkness and into the light." The Doctor gave her a "good luck" kiss before he departed, saying that she would be "just splendid". (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 finding 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 forge an antidote for Mister Sweet's poison, which caused her blindness. She was righteously outraged when the Doctor she found out what her mother had done, calling her a 'perfidious hag' for her deception and betrayal. In her mother's final moments, Ada refused to forgive her to which her mother expressed pride, coldy saying "That's my girl".