The Executioner was a person who recently joined the Shadow Mercenaries. He was someone the old monster, Richter Greyhound, brought back from somewhere, He joined the Shadow Mercenaries, right before James Bourne was supposedly killed. So Oliver had seen the Executioner but only a few times, yet for some reason felt some kind of familiarity with him. Even the others from the Shadow Mercenaries were able to quickly accept the Executioner, not only due to his strength but also for the sense of familiarity he brought with him. It was like he was someone they all knew from a long time ago.
The Executioner himself actually had no idea who he was. When he woke up he was in a lab of sorts, and Richter was the first person he saw. Of course the first question that popped up in his mind, when he got up was, "Who are you? Where am I?.... And Who am I?"
When Richter heard the Executioner ask these questions, he couldn't help but show a sadistic smile on his face.