Psi

 was a hacker / bank robber with an augmented brain who helped the Twelfth Doctor and Clara Oswald rob the Bank of Karabraxos.

Hardly anything was known about Psi. He had previously erased his memories prior to the episode after being convicted as a criminal, and mentioned in doing so, he had erased all his memories of family and friends too. When questioned, he simply replied: "I suppose I loved them." He had a prison code on the back of his neck. (TV: Time Heist)