Psi’s great tragedy stemmed from his ability to wipe his own memories. In order to keep his family and friends safe he deleted his entire recollection of them, leaving himself so alone that when his life flashed before him, he saw no-one. But his misfortune would be reversed, the day he met the Doctor…
Psi is an augmented human with a mainframe in his head which gives him special abilities. He can download and retain vast amounts of information but in the past he’s used his skills for criminal activities, admitting to the Doctor that he was, in his own words, ‘a hacker slash bank robber’.
I am Psi… I have agreed to this memory wipe of my own free will!
He’s strong willed and was initially determined to prove that the Doctor didn’t intimidate him, even threatening to leave the Time Lord’s band of burglars and calling him a lunatic. Some of this hostility lingered and when Clara stood up for the Doctor, Psi chided her. ‘It’s very obvious you’ve been with him for a while,’ he sneered, ‘Because you are really good with the excuses.’
But Psi could also be brave and kind. When Clara was in danger of being hunted down by the Teller, he deliberately drew attention to himself, risking his own life in the process. And before this he revealed he had wiped every detail of his family and friends from his memory, simply to protect them.
The Doctor managed to obtain a rare neophyte circuit which would enable Psi to restore his lost memories. And so the two of them parted as friends, although Psi hadn’t completely changed… As he prepared to leave the TARDIS his final words to the Doctor were a mischievous invitation: ‘If you ever need help with another bank heist…’