Psi was a hacker with an augmented brain who helped the Twelfth Doctor and Clara Oswald rob the Bank of Karabraxos.
Psi had spent some time prison and had a prison code on the back of his neck. At some point he had also been interrogated, and chose to erase his memories of all his family and friends. Unable to remember exactly why, he believed he had done so because "I suppose I loved them." He was able to retrieve the memories he had lost by helping the Doctor at Karabraxos. (TV: Time Heist)