- You may be looking for the Master.
"O" was a member of the British Secret Service agency, MI6. According to the Thirteenth Doctor, he was one of their best. O had previously been a champion sprinter, something which was noted in his official file. However, O was ambushed by the Master before his first day and miniaturised using a Tissue Compression Eliminator. The Master kept his body in a matchbox, which he showed this to the Thirteenth Doctor to confirm his identity. (TV: Spyfall)