Charles Robinson is the only on screen performer that has been with the company since the 2001 buy out. I THINK Johnny Ace has been employed in various roles since 2001, but there might have been a gap after his on screen role where he wasn't working with the company.
Kidman, Noble, Hurricane, Finlay all left and came back. There might be some unknown production people still left, but I'm not sure WWE took on any of the production crew from WCW. I think I remember reading the production team was left dangling while WWE decided on how they were going to run WCW as a separate promotion and most had to move on because they couldn't not work. A lot ended up working with WWA and then TNA. It's possible they hired some of them after the brand split but I don't THINK any came over directly in the buy out.