Remember how in the first series of the first season Six said that Van Kleiss was not a problem until today???
So, I think that he could appear even when Rex was a kid and so I had the idea to draw this moment
When White Knight learned that an unknown (at that moment) enemy wants to steal Rex, he ordered Six to do everything possible so that Rex did not get into the wrong hands...........
But Van Kleiss decided to wait until Rex grows up and begins to run from Providence (I think...)