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Psi was an augmented human who worked as a hacker and bank robber. At one point in time, he was recruited by the Twelfth Doctor to rob the Bank of Karabraxos.
Psi worked as a bank robber and efficient hacker, equipped with an augmented brain. At one point, he was arrested and sent to prison, where he was interrogated. To stop himself from betraying those closest to him, he deleted his memories of all his friends and family, a decision which he ultimately came to regret.
After being released, Psi was recruited by the Doctor to help him rob the Bank of Karabraxos, the most impenetrable bank in the galaxy. Psi, along with the Doctor's companion, Clara, and the mutant human, Saibra, was sent down to the planet and deleted his memories of the last few hours, ensuring that he had no idea what he was doing there or why he was going to rob the bank. The group received a message from the "Architect", who informed them of their mission and gave them no alternative.
Psi and the group successfully entered the bank, using Saibra's shape-shifting powers to get them past the bank's security. In the central lobby, they witnessed the fearsome Teller, a telepathic creature kept enslaved by the bank to detect guilt in another human and turn his brain to "soup". Psi and the gang then proceeded to go down to the lower levels of the bank.
At the vault level, Psi watched as Saibra was forced to use a device they believed would kill her, as an alternative to being killed by the Teller. When the Teller was released into the lower levels, Psi tried to open the vault, but was unable to. When the Teller homed in on Clara, he attracted it towards him using his guilt, and the guilt of all hackers and bank robbers before him, before using the device on himself. Rather than killing him, however, the device merely teleported him to a ship waiting in orbit. He and Saibra returned to the bank and managed to save the Doctor and Clara from the wrath of Ms Delphox.
Going down one last time to the Private Vault, Psi and the others discovered Madame Karabraxos, the director and manager of the bank, as well as the Teller's mate, which had been kept in a cell to ensure the Teller was completely loyal to the bank. They informed Karabraxos of the dangers an impending solar storm would do to the bank, and Karabraxos fled — though not before the Doctor handed her a piece of paper with his number on it and the message, "I'm a time traveller". Years later, consumed by guilt, she would use that number to ask the Doctor to break into the bank at just the right time so he could save the Teller and its mate.
Psi then shared a takeaway with the rest of the gang, and was given a neophyte circuit to give him back the lost memories he so craved. He then returned to his own time and place, as did Saibra and Clara.