The Bad Wolf made Jack Harkness immortal. The Red Room Natasha.

His name isn't Jack yet the first time he meets Natasha.
But he won't remember the meeting, nor the two years after that.

The (other) first time Jack meets Natasha, it's the Cold war and she's killing him, again and again.

The first time Natasha meets Jack, he's pointing a gun at Clint's head.
"Who do you think created the Time Agency, SHIELD boy?"