Madame Karabraxos was the manager of the Bank of Karabraxos. In her elder years, she was the wealthiest person in the universe.
To ensure the security of her bank and the loyalty of all her staff, Karabraxos cloned herself several times, placing the clones in important positions within the bank. These clones, such as Ms. Delphox, were totally obedient and lived in perpetual fear of their master, who had an unfortunate tendency to "fire" those who failed her. She did this by throwing them into the bank's incinerator.
She also kept a fearsome beast known as the Teller, a telepathic creature who was able to detect and get rid of anyone attempting to break into the bank. Karabraxos kept the Teller in her control by keeping its mate, the only other of its kind left in the universe, in a cell below the bank.
While in the position of bank manager, Madame Karabraxos lived in the bank's Private Vault, surrounded by all the wealth she had acquired over the years. At one point, however, a solar storm threatened to destroy the bank and all the wealth inside it. Inside her Vault, Karabraxos was accosted by the Twelfth Doctor and his companion Clara, as well as Psi and Saibra, who had been instructed to carry out a heist. There, Karabraxos was informed of the dangers the solar storm posed to her and fled, gathering the best of her treasures into one small suitcase, leaving the robbers, as well as the Teller and its mate, to die. Just before she left, however, the Doctor gave her a piece of paper with a telephone number that would link directly to his TARDIS and the words, "I'm a time traveller".
Years later, Karabraxos was haunted by regret after leaving the Teller to die. Finally using the number given her to contact the Doctor, she asked him to break into the bank and to save the Teller, taking them to another planet so they could live in solitude and begin again. The Doctor quickly accepted this mission, and so proceeded to travel to the bank just before the solar storm hit. (TV: Time Heist)