It's certainly not any character I have ever heard of. If such a character did exist it would make all Tau-based death stars completely immune to melee, which they are not. Maybe your opponent just has a poor grasp of how Hit & Run works?
There is a character named darkstrider who can join firewarriors or pathfinders that allows the unit to consolidate D6" after firing overwatch. This is the closest thing I can find to being close to that, or it's like Casiarius said and he doesn't understand how hit and run or assualt jet pack moves work.
Thanks for the help..... I think he mistook the consolidate D6" after firing overwatch as to be a assault jet pack move. I need to look up the consolidate rules as to how that should affect his distance of move also. He was moving thru difficult terrain. Possibly since he's Tau he could ignore difficult terrain?
It just surprised me that any unit could move after firing overwatch but before I can roll my assault distance.
He is a new player and I was playing my Grey Knights (3rd game with them).
Either case it wasn't a game breaker or anything. I just need to be aware of that 'Darkstrider' character and to possibly surround him with multiple units if I want to assault him.