O (Spyfall)

"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 and killed by before his first day, being miniaturised using a new and improved Tissue Compression Eliminator.

The Master kept "O"'s body in a matchbox, which he showed to the Thirteenth Doctor to confirm his identity. He stole O's identity and set himself up in MI6 in his name, maintaining the charade even as far as O's retirement to the Outback, before eventually revealing himself to the Thirteenth Doctor as his Kasaavin masterplan reached completion. (TV: Spyfall)

Appearances
Not unlike his killer, "O" had short black hair. (TV: Spyfall)