'The Teller' was the name given to a member of a telepathic alien species, that can scan thoughts and emotions, including guilt. It was also capable of emitting a form of energy that could liquefy the brain, causing the crown of the head to "cave in"
It was one of only two of its species left alive. It was forced to work under Madame Karabraxos and her clone, Ms Delphox, at the Bank of Karabraxos, as it's mate was held hostage. Just before the bank was destroyed by a solar storm, when nearly everyone else had fled, the Twelfth Doctor freed both creatures. He then dropped the reunited pair off together on a remote planet, far from civilisation so that the thoughts of other beings could not torture them.