Maybe people should by a strategy guide because in there it tells how to put all of the ghosts to rest. It does not say however what happens if you already have Windwalker at the beginning and you finish the Ghostbreakers level. Is he there or what? ;D
Windwalker couldn't be on Unusual Suspects. You can use him there. However, you can't use Windwalker on a return to Ghost Breakers. Maybe he's hidden a little deeper within the building for people who couldn't get him the first time round?
For those of you that did not purchase a Strategy Guide it says in there that because Windwalker could have been recuited at the beginning of the game if you got him by other means that he will be available in ALL Time Gate Hauntings. So he will be available in Ghostbreakers if you got him in that part and then went back to the Time Gate.
When you have a pesky mortal that needs to be scared. Who are you going to call? Ghost Masters!